Longbow55
I sent you a PM.
There are various tips on how to get into the airways system unambiguously.
One is to file an ICAO flight plan (you have to anyway, for Class A) with a realistic level, e.g. something like FL100-150 on it. If you put 2400ft on it, UK ATC will probably think "this is an IMC Rated pilot trying to be ultra diligent" and they will just address it as per VFR.
Another is to TELL the tower to phone up London Control and get an airways clearance for you.
But it does sometimes happen that ATC keeps you down low for a long distance. I don't have experience of this personally since I have not flown airways on what I regard as trivial flights within the UK which can be conducted on the usual informal VFR-come-IFR basis. But I know of very high hour IFR pilots who find this, and what they tend to do is they fly the "informal IFR" within the UK and change over to airways once leaving the UK.
The whole point of an IR is for IFR into Europe, anyway (in the piston aircraft context).
It isn't ATC's job to question pilot privileges, so the N number will mean nothing to them. A lot of pilots fly N-reg on the IMC Rating - do you think every one of the 50 or so Cirrus owners in the UK has a full IR?