hey Chris,
unlikely to be UK suppliers, you could give
www.realsteel.co.uk a try though?? Or try get some LT4 heads from a vette these are a good upgrade as far as i'm aware, but you'd have to read up about it, im not so clued up on the LT1
If not try this dude,
http://www.elliottsportworks.com/lt1.html, he gave me a reasonable shipping quote in the past when i was looking at heads/cam but realize the auto box wont be upto 400+ hp, plus i wanted to do my exhaust first. If not texas speed performance do good quote too and reasonable prices
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'99 Z28 A4, 253 LS2 heads, 228R TSP cam, Patriol Dual Springs, Double roller timing chain, LS6 ported oil pump, OBX LTs, 3" True Duals, LS6 intake, 42lb/hr injectors, SLP lid + bellow, EFIlive COS 3 Speed Denisty Tune (self tuned), 3200 Circle D stall, CAI mod, KYB AGXs, Eibach Pro Front Springs, UMI panhard, LCA + Brackets, Strut Brace, Custom Subframe connectors, WS6 axle with 3.42s, 18" American Racing Rebels, US mirrors, 93 tail lights, 3.5 inch custom cowl hood. Mickey T's Street DRs on Vette-ZR1 wheels for the strip !
Best ET: 12.0 @ 116 mph (on a 0-60ft of 1.8 s)