This does strike as rather unusual - I've missed the outward flight of a £59 return EDI-LHR and BA offered a 'special' charge of £80 to go on the later flight for passengers on non-flexible bookings who missed their flights (presumably it was discretionary).

In my case we declined the offer as it would have required a couple of hours wait - the next BA shuttle was full. Went with Scotairways instead.

Would certainly follow your case up with BA as think they have been quite unreasonable - esp. if there was availability and on such a highly competed route as BRU.