2 friends (BA blue) recently were donwgraded on the rturn leg of a 2-4-1 from F to J BGI-LHR

F seats (same day as orignal flight) came available for Avios whilst away on holiday in Barbados; called to upgrade quoted extra Avios of 25k and taxes of £267 x 2 (which seems high to me?) and confirmed that extra charge would come off the CC used for original J booking. Both were able to reserve 1E/F on 787-10
At check-in at BGI unfortunately staff had issues and initially they couldn’t find booking and when they did find it said booking was still in J. After over an hour of waiting eventually were given seats in J (Expertflyer told me ealier in the day that 1E/F were the only 2 seats taken in F and J was full, subsequebtly I checked EF and F went with 7 out of 8 seats taken. On-board cabin crew also confirmed that all 7 were upgarded to F).

Any ideas what could have gone wrong here and what compensation is due?
TIA