I will echo Saladman's welcome to FlyerTalk, even though it is unhappy circumstances that have brought you here.

Firstly, BA is currently overwhelmed with customer issues, I am reading of response times of 4 months and more. If they have not yet given you a response to your complaint, there is some chance they may agree to extend your Bronze status once they get round to processing that complaint. There is nothing you can do to make processing of your complaint happen more quickly.

Secondly, while I hope you get a positive outcome, my inclination is that they may stand firm. The issue is that you didn't get 300 TP, and once you opted to refund your flight, that was the end of BA's obligations to you. It is of course easy to say with hindsight, but had you found FlyerTalk beforehand you may have gotten different advice. If you had kept the ticket alive and worked out a way for BA to reroute you, you would have been able to claim what is known as "Original Routing Credit" or "ORC", where BA award the TPs for flights you couldn't take because of their issue. I know BA can even reroute onto the railways (though not sure if ultimately to MAN). I appreciate you had a business meeting, but it may have been better to claim the expense for alternate transport afterwards rather than cancel the ticket. Since BA refunded the ticket, their view may be that you won't get any TPs, but it's certainly worth a try.

Lastly, and I don't mean this as any criticism because I've been very guilty of this myself, try to shake the mindset that BAEC is a reward for loyalty. I know they present it as such, and for all sorts of obvious reasons related to psychology, we think of loyalty as a positive trait. However, as one of the sages of this parish has been known to observe, loyalty is for dogs, not airline passengers. The purpose of BAEC is to drive your future business to them, and if you feel they have given you no reason to do so you should absolutely consider all the alternatives. They won't care either way, and you may well be better off.