The BAA strictness varies from month to month. At its worst they were very strict, and extra 1/4 inch led to the bag being banned (this was in August 2006). I witnessed this several times, and once in the F lounge came across a BA Premier who was furious and taking it out on BA SS: he had arrived at LHR with normal carry-on, which was too large (by an inch), so he went to the luggage store and bought a new bag (for GBP 300) that was sold as being within the BAA limits, repacked and checked original bag, carried the new bag to security. Rejected again as too big. So he had to check that new bag too. The punch line: it was an inch smaller than the size limit published on the BA web site, hence the argument with BA. And this was for a Premier! I suspect he was going to be using the VIP security line from then on instead of Fast Track.

So, when the next security alert comes, you can be sure that the 22 inch Travelpro will not be allowed as carryon out of LHR. They do this to reduce the security checking workload (fewer bags to check, less volume to check). It is an absurd situation and very damaging to BA.