Contacttower
Sorry, but it appears that IO540 is massively exaggerating the difficulty of the issue.
What you want to do is perfectly normal, I have done it many times. Yes you will have to leave controlled airspace, but will then be allowed back into controlled airspace if that is needed. Flying around Europe in an unpressurised aircraft in the airways is not a problem in most cases. Very occasionally there are difficult areas, where the convenient airway is too high and controllers will not clear you to leave controlled airspace, but it is very rare.
As for basic service being useless, IO540 appears to be flying on a completely different planet to the rest of us. I have heard aircraft reports that put me in conflict (estimating same VOR, same time, same altitude) and was able to find further details and spot the aircraft. I have many times been given useful information by the London Info, and I know they keep some sort of eye on the radar. It is kind of a hint when I change heading to avoid a danger area, and they check what I am doing, and let me know it is not active!
The lowest levels at which one can get LTMA crossings are around FL100 plus
Sorry IO540, that's balls. Most of the LTMA is available from FL70 or FL80, avoiding the SIDs with FL60 limits. Never been asked to cross LTMA above FL100, although been offered better routing by going higher.
Cameltoe's experience of N864 is the same as mine, and flying up to Scotland during the week is much the same. Get a service from Warton, as there can be some military activity, and you get good warning to have a good look at the pointy aeroplanes.
If you want any specific advice CT from someone who has flown hundred of hours IFR all across Europe in unpressurised aircraft drop me a PM.