Not aware of any British car-specific events. There are always a bunch of car things going on, including Exotics at Redmond Town Center every Saturday morning. If you want to see people drive in with cars that are straight out of magazines (supercars, Ferraris, Lamborghinis, Porsches, Lotus, classics...) then this is the event. It's just across Lake Washington from Seattle in Redmond (about four miles east of Microsoft's campus.) Usually a great mix of people and cars. If you search online you'll find a bunch about it.