What you describe doing is called hidden city ticketing. For various and complex reasons that I won't go into here, airlines prohibit passengers from skipping segments on itineraries.

I would strongly advise against simply not showing up for your Dublin to London flight and attempting to check in in London. Doing so will very likely cause your onward flights from London to be canceled.

I know it seems kind of dumb to have to do this, but if you can find a London to Dublin flight for cheaper than your change fees with BA will total, I would suggest doing that. It's all right to roll your eyes at BA for this (and call them out on Twitter or Facebook for being environmentally unfriendly ), but they aren't doing something that every other airline on earth doesn't also do.