Not in the UK but since most everything is imported I will assume you have the same issues there.
I work on a few cars and boats and the quality of replacement aftermarket parts is terrible. It is not uncommon for brand new parts to not work of fail shortly after being installed. Aftermarket brands are the worst! I am to the point I gladly pay for the factory replacements because the quality is better.
Me, I would buy the factory replacement. While I agree that factory replacement parts are high, overpriced even. There is a reason the aftermarket part is $20 too.