IF they've been out of the UK for 14 days then there is a AA/BA option via Miami on Thursday albeit they'd then have to quarantine on arrival (EDIT: and need a PCR test as they'd be arriving Friday, so this is likely a non-starter!). Otherwise it depends if BA can book into an
InterCaribbean service to Barbados - looks like is availability on Friday. Otherwise they can either get rebooked from Barbados themselves and then buy the connection separately and try pursue BA or stay and enjoy BAs 200 quid a night hospitality which I'd be very tempted to do right now
EDIT : found the guidline now on BA Trade- there is a reroute option onto Caribbean airlines -
https://www.britishairways.com/en-gb...kba?faqid=8525 however the next flight available in the correct class seems to be to Barbados on the 24th...