Airstairs, or a stair truck are provided by the ground handler, whoever that may be. The fact this is LHR puzzles me a bit that BA wasn't responsible for it, but if you were at a spoke station I would totally get it.
As a ground handler this stuff happens sometimes. I have to get 2 pilots onto an Embraer 175. My company has no stairs, we don't need to have them. I rented/borrowed stairs from a company I used to work for, those stairs are for 737 and up (I guess) but they wouldn't get low enough. In the end I told the pilots they'd have to board the plane via belt loader, and they did.
I'm not sure I understand what you meant by the crew "crashed the jetty". Do you mean the bridge agent crashed the bridge into the aircraft?
I'm also confused cuz it seems they told you that it wasn't their issue. Well it depends on who you talked to. If you asked a ramp guy like me why the bridge crashed into the plane I'd say something like "sorry, I don't know I don't have anything to do with it". If you ask a bridge agent where the ramp stairs are, they'd tell you the same thing, because they often aren't interchangeable jobs.
In the end it sounds like you had a little bad luck and are a bit sour about it. I'd just forget it.