englishal
Certainly somebody makes a lot of margin on the USA -> UK import of a lot of stuff, but it isn't (usually) the local maintenance shop. It is the importer, usually a sole agent, who brings the stuff in that makes the 1.5x markup.
If you have an N-reg plane, or to a lesser degree G-reg Private CofA one, you can buy parts mail order from the USA, and they are exactly the same parts which are 2x the price here and they come with the same original manufacturer's CofC. I pay 50% extra for fine-wire spark plugs, about £200 extra in all, as a result of having a G-reg on a Transport CofA. The CAA requirement for JAR145 documents on major items and 8130-3 on just about everything else, and training planes being on a Transport CofA, keeps this tight supply chain (a "cartel" in any other language) in business. The official UK importer is a JAR145 company, of course. This is just the tip of the rip-off iceberg.
A lot of the extra cost is unavoidable; e.g. 1 packet of 10 gaskets costing £50 gets shipped over by UPS who charge £50 for the carriage. That's a 2x price increase right away.
One cannot usefully compare UK rental pricing with USA pricing however. In this business, it depends so much on volume and in the UK the same volume just isn't there. Land costs are usually far lower there, there's a lot less pressure from NIMBYs and from property developers.
A lot of stuff in GA is grossly overpriced anyway; avionics is a prime example and that one is a result of most of it being made in small quantities by large firms with large fixed costs. They blame it on certification costs but that's bunk. But due to the age of the UK fleet, the market for avionics here is very small indeed which makes the situation even worse...