A close friend of mine was on this flight and have given these accounts from conversations overheard and some observations during boarding. Could this be the reason why BA responded in the way they did...?

1) The OP was informed that his/her Club World seat had a broken foot rest and would be downgraded to World Traveller Plus.
2) The OP accepted the World Traveller Plus seat and the usual compensation which would be offered in an involuntary downgrade scenario.
3) The OP was also invited to use the broken Club World seat (despite the involuntary downgrade) during the flight for the meal service or for any extra comfort.
4) Once onboard the OP was not happy when he/her learnt that the foot rest in the Club World seat was completely removed (not broken as he/her was previously informed).
5) The OP chose to voluntary offload from the flight as he/her was not happy with the options offered onboard.

A very unfortunate experience for the OP and I understand why he/her would be upset. I'm sorry that I don't have any further information to offer but thought this may explain BA's harsh service recovery actions.