Britpart are just a reseller of other peoples stuff - some of it is OK, some of it is carp - you have to look at what you are getting for your money - for example a britpart drop arm complete with ball joint is £26+VAT. The OE (adwest, not LR branded) ball joint alone is about £20 on it's own. So assuming paddock are making a reasonable margin, as well as britpart, the cost of manufacture of that safety critical part is probably about a tenner, including joint. Sounds suspiciously cheap to me..
For something like brake discs, the price difference between britpart and mintex isn't so much, and you know they have to be made to a BS number. I'd say if you took the BP and the Mintex ones out of the box you'd be hard pressed to tell the difference - they prob both come from the same factory...