I use these guys:
Tadworth Tyre Specialists Ltd
2a, Shelvers Hill
Tadworth Surrey
KT20 5PU
and take the tyres down to 3mm. They charge something like £6 a corner and are nice guys.
However if you happen to be after 195/45/15's on peugeot 205 rims, i have a couple i'm trying to sell
(two sets of yoko 539's and two goodyear f1's all down to 3mm)
jon