Yup - the primary problem is BAA (the shopping mall company masquerading as an airport authority). The airlines have sizers to try to minimize the number of people sent back.

Having said that, the checking at security seems to be a bit less strict at Fast Track (at least from T3 landside where I last did it) than in the main line. If you are in J out of LHR you should be fast track.

(Not that this matters to the OP, who isn't going to the UK)

And, by the way, I'm not sure where the OP has got this idea of "international carryon" from. The US has essentially the same restrictions - less than 45 inches in total for carry on and it must fit under a seat or in an overhead locker (see, eg, https://www.aa.com/content/travelInf...lowance.jhtml), which isn't much different from 22*14*9.