My wife and I are going to be on the Britian Sampler in a couple weeks and were wondering how strict the guide and driver will be with the size of our hand bags. We would like to take something larger (18x12x7) than the small complimentary (12x11x6) bag provided.

Your bag should be fine. There isn't a whole lot of space above the seats so that is why they request that folks don't bring on bags with wheels. It is doubtful they would fit overhead.

I brought on a slightly larger bag last year and had no problems.