Alfa Romeo Giulia Quadrifoglio: Thought about this while in Italy. The Carabinieri had a couple of Lotus Evoras, so why not give British cops an Italian cop car?
2015 Audi RS6 Avant: This is one I am excited about. Fast, grippy and its credentials as a getaway car are already well-established. British police have used a few A6 Avants, so it fits on both sides of the law as well.
2013 Audi RS4 Avant: Like the RS6, the RS4 is an ideal choice for a pursuit car, especially when winter comes. The Audi A4 has also been used by British police. Not as common as BMW 3-and-5 Series, and likely already completely replaced by said BMWs, but they have been out there.
Audi RS5: One of these made headlines a few years back for hitting 180mph while trying to escape police. Lamborghini engine or not, an ideal getaway car?
BMW M2: I plan to add one of these to my police fleet. It may not start out that way...
BMW X6 M: The BMW X5 is the most widely-used SUV in British police service, for both Traffic and Armed Response duties. I'd prefer an F15 X5 to an X6, but the X6 M is a capable vehicle that I've already used in anger. I'm sure
@JR98 remembers the Rhino chase in Surfers Paradise...
BMW M4: I'm looking to BMW to make up a sizeable part of my police fleet, just like in reality. And just like the 3 Series, the M4 ought to make a capable pursuit car.
BMW M5 F10: There's a reason I chose this to showcase my first livery draft: the F10 5 Series is one of the most common large sedans/wagons used by British police.
BMW X5 M: The E70 X5 has seen a lot of service with police forces, and likely continues to do so alongside the newer F15s. They can carry a lot of stuff and can move just like the BMW sedans. They're absolute tanks as well; one X5 was reverse-rammed by a flatbed truck, crunching in the front-end, yet was able to continue pursuit for another 20 miles.
BMW M5 E60: Another one I am excited for. While the E60 5 Series has now been almost completely replaced by the F10 and G30 models, it wouldn't surprise me to see an old E60 still hanging around in police fleets.
Cadillac XTS Limo: I expect to see at least one of these being chased. If not, I'll be very disappointed...
Chevrolet Corvette Z06 C7: You can actually buy a C7 Corvette or a 6th Gen Camaro in the UK. Admittedly, they are left-hand drive in a right-hand drive country, and there's only a handful of dealers, but there is still a way...
Ford Ranger Raptor: Not on the list, but confirmed to be coming, and I am very excited for this. The Ford Ranger is a popular choice with police forces looking for an off-road patrol vehicle, and the Raptor version promises to be the most capable Ranger ever.
2018 Ford Mustang GT: There was actually a marked police Mustang demonstrator running around the UK a couple of years back, and a black Mustang GT was sent to various police forces for evaluation. It's all gone quiet on that front, so I can only assume that it was deemed awesome but impractical. Of course, we don't have to worry about impracticality...
2017 Ford Focus RS: Like the Mustang, the Focus RS was also turned into a marked demonstrator. Fast, agile and with a few Focus STs used by some forces, so an RS is the logical next step.
2017 Ford F-150 Raptor: Clive Sutton Cars will import an F-150 Raptor to the UK for you. They'll even give it a right-hand drive conversion should you so wish...
HSV GTS and Maloo: I would just like to point out that the HSV GTS was sold in Britain as the Vauxhall VXR8. This also extends to the Maloo ute.
Jaguar F-Pace S: Another marked demonstrator that I feel has potential, especially in supercharged V6 S spec. A smaller, more agile kind of Rhino.
Jaguar XFR-S: I believe it was Central Motorway Policing Group that had a few Jaguar XFs as patrol cars. Good enough for me.
Jaguar XE S: Not actually used as a police car, but an XE with the supercharged V6 would fit right in.
Jeep Grand Cherokee SRT and Trackhawk: Jeep sells the Grand Cherokee in the UK. Hilariously, this includes the SRT version. Even more hilariously, I am led to believe the supercharged Trackhawk will also make it to Britain soon.
Kia Stinger: Bit of a strange choice, but hear me out. First, the Stinger has performance, with a twin-turbo V6 and agile rear-wheel drive chassis. Second, I got the chance to look around a Stinger at a dealer, and I think that it definitely has potential. Finally, it has pedigree, with the Queensland Highway Patrol selecting it to replace their Commodores and Falcons.
Range Rover Sport SVR: Carrying on a grand tradition of Range Rovers serving as police cars, the Range Rover Sport SVR is at home assaulting a mountain road, assaulting a mountain, charging through a field and looking pretty outside a department store.
McLaren 650S: A 650S with police livery was shown at the Goodwood Festival of Speed one year. That's all the justification I need.
Mercedes-Benz Unimog U 5023: I knew it as soon as I saw it. You know it. This potential new SWAT truck is ready for business.
Mercedes-Benz A45 AMG: Another car which isn't actually a police car, but I'll give this a try.
Mitsubishi Lancer Ev… Hang on a 🤬 minute...
Porsche 911 Turbo S 991: A police liveried 911 Turbo was also shown off at Goodwood. Something tells me you guys aren't even going to need this level of justification.
2015 Subaru WRX STI: The latest in a long line of rally cars for the road, the STI's performance and all-weather, all-terrain capability ought to see it continue to be a favorite.
2005 Subaru Impreza WRX STI: The hawkeye Impreza should be familiar to anyone who watched the early seasons of Police Interceptors, as it was a mainstay of the Essex Police ANPR Interceptor fleet, along with that other rally-car-that-shall-no-longer-be-named...
Terradyne Gurkha LAPV: Although I'm excited to see what the Unimog can do, it's still good to see our original Big Boi return.
Toyota Land Cruiser and Hilux Arctic Trucks: Beefed-up, off-road modified Toyotas that should be very capable in the mountainous areas.
Vauxhall Lotus Carlton: An oldie, but a goldie. The Lotus Carlton was a favorite of thieves, who would ram-raid shops and steal whatever they could carry, before making their getaway in the Carlton at speeds the panda cars could not hope to match. There is at least one recorded case of this actually happening.
Volkswagen Golf R Mk7: There is evidence to suggest at least two police forces use the Golf R as unmarked vehicles. And why not? It's a very capable hot hatch that doesn't shout about its performance too much.
Volvo V60 Polestar: British police have used Volvo wagons for more than 50 years. What better way to carry on that tradition than with a V60 Polestar? I have no idea how they got a transverse-mounted turbo straight-six in there, but I'm kinda glad they did.
(I really would've liked to have seen the XC90 T8 in the list though...)