If you have an UK Spec Auto already, then keep the Gearbox, get a VVTI Lump and a Syvecs. The EU/UK A343E Gearboxes are stronger compared to the Jspec ones. Both gearboxes' Valve Bodies can be modified for harder and faster Shifts too. Combined with a Syvecs and Paddles you'll have a much better, faster and more fun to drive result than with just swapping stock parts over.
As for UK spec vs Jspec Turbos. I'd just get the Turbos rebuilt with steel Turbines. Then the Jspec Units could make more power than EU/UK Units because they are a bit bigger.