Generally if I have a choice I go for smaller companies doing SPECIALIST things, much more interesting and generally less chance of being outsourced, I don't think I have ever run across a company faking it in the way you describe, it is just not a thing that happens in my experience.
I have no idea where you are finding those companies, in my experience generally an importer is usually quite up front about it, and we love them for it because they are in this time zone and deal with the QA and customs wankery (That is their value add). The good ones may even have a few engineers on staff, sales support, and dealing with the customer interface to whoever is making the stuff on semi custom work, that kind of thing.
You tell by asking questions at interview, which is very much a two way process if you do it right.
Now subassemblies from China, yea sure, but that's just because no bugger in our office wants to do the design and certification work for something like a 24V 30W power supply, I mean sure we could, but why bother when it is like £7 from Meanwell, and not that much more from TDK. We all got things to do that actually add value to £10k plus per unit products.
BTW: We found something interesting, for contract PCA assembly, at the 500-1,000 units/year sort of volumes, UK is actually cost competitive with the far east.
Metalwork however needed to go overseas (Poland if I recall) to get the right sort of numbers.
Regards, Dan.