I think the biggest problem with Big Ben is, that it's a symbol of the UK for foreigners, but less so for british people, and even less so for those in Wales, NI and Scotland.
You never know, they might take something from this, i'm not really sure on the thinking of the judges, but the fact that Big Ben has never appeared on UK coins suggests to me that they might not do it this time either.
The problem now is you have the Welsh Assembly and the Scottish Parliament too. There's talk of reducing the power Scottish MPs have in Westminster and so on. How well would this coin make it in the future?
The fact that it is so debatable, is it's problem.
Look at previous designs, you have bridges, flowers, national symbols, that's about it. Britannia is probably the best symbol of the UK as a whole, their "no people" rule covers this or now? I doubt it, so maybe they'll go for Britannia again, the mint probably doesn't want this based on their silver and gold coins, but the mint isn't choosing the design, I would have thought and certainly doesn't have last say (the queen does).
I'm hoping something a bit different comes along, either with these but not looking traditional, or something really different. But I think buildings will not appear unless you can get 4 on a coin, and i think this coin will be too small for that.