i guess its along the lines of a lot of mods, if done well, they look so much like they are original, that only a trainspotter would know, at the other end of the scale but same story, you have E30 M5x swaps, no custom parts needed except and exhaust, its all nuts and bolts, so how would they ever know.
Personally, im more worried about Chinese tyres and blown shocks being legal on our roads than the odd Jag enthusiest retrofitting a leaping cat on the front