I've used Bitstamp a couple of times this year, but I've only sold crypto. The SEPA withdraw works nicely and also
the explanations for SEPA deposits sound pretty straightforward.
Unfortunately I don't remember what papers they've needed for KYC (and if they were more than just the ID).
And it may worth knowing that Bitstamp works with USD or EUR, so probably your friend's GBP will suffer a conversion at his bank's rate first.
Edit: I know it's not UK exchange, but from my experience there's a good chance the final price will be better than on a smaller "local" exchange.