Throttlebodies wise Matt only does them for the CG series engines and they require additional work to get them to fit, it's not a bolt on affair.
Most of the parts for the GA16's are made in the US, as the engine is more widespread and tuned out there. Hotshot do manifolds, although you can get cheaper stainless ones somewhere within the UK i think. Anything serious like cams, flywheels etc it's going to be from the states and can usually obtained from JWT, as they have a good range of performance upgrades. The car to look for is the Nissan Sentra 1.6.
Just remembered their engines have variable valve timing and I never got a difinitive answer from the Primera Owners if the cams were ok in the UK spec engines.
One up side is the dollar is good right now.