http://www.htbl.co.uk/find-us/ These folk in Bradford fitted a new hard drive and battery in my ibook a few years back. The service was good and the price seemed fair. Someone had recommended them to me, so I reckon my experience wasn’t a one-off. Was years ago, mind. Good luck.
Apple do not sell their parts, any company offering you a genuine Apple screen is likely hoping you cannot tell the difference between the fake part and the Apple original. They are using the genuine screen fallacy to scam
It’s actually a pretty easy job to do yourself if you have some technical ability ( or just careful )
I’d imagine it’s just the digitizer £20 – bezel £4 – and possibly a wifi ariel £3 if you cut it when removing the cracked screen.
Plenty of guides are available online to follow.
If it’s a recent ish model I would have thought the extra cost in having it repaired properly by Apple would be outweighed by it being easier to sell on later. Might be worth asking Apple to check their prices anyway.
If you think you might keep it for a long time it might not be as much of an issue.