Here are the UK 2004 sales figures. It would be interesting to have "Other UK" broken down further really: (obviously this is UK sales, not production)

Ford 367,158
Vauxhall 325,012
Renault 189,342
Volkswagen 181,897
Peugeot 167,822
Toyota 121,081
Citroen 105,596
BMW 102,200
Honda 91,241
Nissan 90,223
Mercedes-Benz 81,963
Audi 77,039
Fiat 72,825
Mazda 47,739
Rover 44,779
MINI 43,660
Land Rover 41,401
Volvo 40,005
Hyundai 37,611
Skoda 35,029
Suzuki 33,656
Kia 33,419
Seat 33,228
Jaguar/Daimler 32,535
MG 31,989
Mitsubishi 22,573
Saab 20,294
Daewoo 17,560
smart 11,844
Lexus 10,047
Chrysler 9,312
Subaru 8,823
Porsche 8,622
Alfa Romeo 8,072
Jeep 6,918
Daihatsu 4,141
Bentley 1,993
Proton 1,754
Other Imports 1,640
Other British 1,351
Ssangyong 928
Lotus 879
Perodua 760
Aston Martin 636
Isuzu 510
Tata 162
Total 2,567,269