Plenty of Class A airspace in the UK still. The LTMA is one, along with the Worthing CTA.
In my understanding, and bear in mind I'm an en-route controller, is that you can cross an airway at the base level, but crossing the base of a CTA at the base level will mean you're infringing. You need to be below it - I think.
I'm prepared to be corrected on that by someone who operates a lot lower to the ground than I do most of the time, but that's what I understand it to be