Use Co-operative bank! They are reportedly one of the only banks to be "OK" with SEPA despoits/withdrawls to exchanges. Natwest for example now don't accept this, and other banks have been questioning these transactions to their users. Don't bother wasting your time having a shit bank to deal with. Just use co-op to channel the money through.
I've been sending money in and out for months and they've never questioned me

For anyone else with this complication, it can be a bit tricky to understand to begin with, with the SEPA transfers and converting to EUR (physically and mentally!) but you'll get there. In the end it's worth it as there's so little volume of BTC/ETH/LTC in GBP (10% compared to Euro trading), so you'll get a much better trade out of it. Not to mention GDAX only have BTC/GBP as a GBP trading pair.
I'm still waiting for BCH/EUR to go live though

Am thinking US users are thinking

as they don't have these problems between crypto and banks over there...