Generally, nowadays most banks will not even allow you to swap a £50 note into £5 and £10's, unless the cashier knows you personally. I know of a few businesses who frquently require £100 petty cash notes changed into lower denomination. It is such a pain in the arse (the petty cash girl is not a signatory on the company bank account), so she ends up paying it into her account, and the bank then gives her equivalent amount in bags of coins.
Jimmy is correct in that Barclays will not allow you to simply walk in with £500 in old notes, and do a straight swap for £500 in new notes. Whilst they would still let you swap them, they would insist that you pay into your account and then take it out as cash immediately. This way, they have stayed within the laws of Money Laundering Act. It also ensures that only customers of Barclays can use this service.