is there a safe way to sell off 20btc's into a UK bank account without a chance the
bank account is frozen ?
I think it depends really about your client history.
If you are new client sand didn't have so big transactons before, it could be considered a suspicious transaction and the bank could block it and ask for more information from you.
If you are a long time client and you've already had such great transactions before, the bank is unlikely to make any problems.
I suggest, talk to your bank official.