There's 5 different qualities of parts:
1. Genuine Land Rover
2. OEM
3. OEM specification
4. decent budget specification
5. cheapest
There's no difference in quality between the genuine LR and OEM part. Basically, they're the same thing, but labelled/packaged differently. There will often (but not always) be a massive price difference though.
OEM specification are parts made to meet the same specification as OEM/genuine, but won't actually be the same maker as the original parts supplier. Typically these are well known quality brand names such as FAG, Hella, Sachs, Bosal, ATE, Mintex, etc etc
Decent budget specification might be slightly lower quality but tend to be alright. These are the kinds of parts which a reputable auto factor might stock, and supply when an owner (or more typically, garage doing work for an owner) asks for the "cheapest" rather than a top quality part.
Then there's the bottom of the market, which are (typically) very cheap parts made in far flung places such as China, Vietnam, India etc and are normally of poorer quality than most reputable suppliers would be happy, also garages dislike these parts because if fitted, the garage is responsible for the work done INCLUDING the part, not the parts supplier at the end of the chain. These are the ones which are normally a bugger to fit, come with the wrong bolts, only last a few weeks-6 months if they're rubber, have imperfections, etc.
Britpart don't physically manufacture parts themselves, they buy from a large range of parts manufacturers/suppliers which might be 2-5 on the list (typically 5, but some items are a bit better than that).
