Yes, it was one of the angled balcony cabins on Deck 7 which I selected, assuming - because I'd already chosen Britannia Club as my fare - that it was a Club cabin (i.e. Grade A1 or A2).
Also, I've now noticed that one can almost make the same "mistake" on Cunard's own site: selecting Britannia Club, then the option to choose the cabin for oneself allowed me to choose a cabin in the BB grade; only in this case both the name of the fare at the top of the page and the price to pay updated accordingly. If one was paying attention it'd be clear that the product had changed. This didn't happen on the TA's own website and the agent I spoke with this morning was able to see the problem for herself.
Why it's possible to select a cabin grade which is not in the fare class is beyond me... but anyway: thanks again for all the advice. I expect it'll all get sorted sooner rather than later, and I'll end up with an A2 Club cabin (as that's what I'm paying for). I'm sure it will be lovely... just perhaps not quite as exciting as I first thought!