Doesn't it have to be registered, and the money paid to a bank account
later

AIUI yes - to discourage theft of them for the deposit/balance on them.

I *think* the position for the separate "Visitor Oyster card " is
different with tube stations refunding up to £5 when the card is handed
over. But I can't find an answer one way or the other on the Oyster
pages.

Won't there be a bit of a long queue at Heathrow tube station
(or wherever is the OP's point of final exit from the "TFL system".

Good point: I really don't think it'd be worth the OP's time bearing in
mind (i) how far the pound has slumped against the US dollar and (ii)
how much we (UK PLC) are broke and need the money.

--
Robin
PM may be sent to rbw0{at}hotmail{dot}com