Sainsbury's used to let you turn the voucher into cash at any checkout, but now you have to queue up at the customer service desk to do it. Be warned: If it's over £50 they have to get it authorised by a supervisor, which normally adds another few minutes wait while they try and find one. If I...
I assume you have to have an account with the bank in question. I always use the machines in Asda/Tesco/Sainsbury's to change up coins, great though they are that 8.9% commission they take is a bit annoying.