Ok, so basically, you cannot fly IFR through controlled aispace without conforming to the airways/ATS routes and their lower limits?
Well I'd put it more bluntly than that. You need to be at at least FL90 to fly through the TMA without causing a lot of extra work. Even at FL90/100 you may well cause some extra work for ATC. There are so many DCT segments of standard routes and the filed route is so infrequently flown that any airway MEAs are almost irrelevant. There is a minimum level for
overflying the London TMA, but I can't find where that's published.
I thought that below FL245 you should fly at a level applicable to the semi-circ rule?
The semicircular
rule applies outside controlled airspace, and these days is pretty much irrelevant. However, you should probably file a semicircular level where possible, and fly the level assigned by ATC.