Craig at Dyno Torque did a Supra a few years ago http://www.mkivsupra.net/vbb/showthread.php?222367-ls1-supra
It's not easy to find a used LS engine in the UK, so you'd have the hassle of importing from the US. LS crate engines seem like good value, but a turnkey conversion will cost you £5K+ on top of the engine, gearbox, flywheel, clutch, ECU, and ancillaries.
GM crate engines and gearboxes in the UK http://www.partsworldperformance.com/
The UZ has the advantage of dropping straight in, but a manual conversion would cost, and unless you can fabricate a turbo installation it will be expensive to increase the performance. However, if you have the skill it looks like a comparatively easy job once you have reversed the standard exhaust manifolds to point forward.
A small-block Chevy would be the cheapest to tune, but it's heavy (don't know how much CW's alloy block would cost ), and you might not want to suffer a carburettor after years of nice convenient EFI, so you'd need to budget for fuel injection. Also has the advantage that an old enough block would't need a cat.
A small-block Ford is a little smaller than the Chevy, but they are both smaller than a UZ, so there's no advantage in reality. Apparently, the OHC Ford "modular" engine is expensive to mod.