You should not be denied access to CAS if you file the FP at a decisively high level so there is no possibility of mis-understanding.
Some pilots have filed IFR FPs for say FL050, and found that London Control would not talk to them - even though the FP would have taken them into Class A CAS pretty soon.
I don't know the limits on this, and multiple enquiries over the years have never established what rules London Control apply to accept/reject a flight plan, but it could be that the minimum sensible initial filing level should be about FL090. Personally I always file at/above this and usually FL140 because one usually gets much better routings up there.
However, if departing from further up north, where CAS base might be FL105, one's filing level needs to be even higher, otherwise LC (or Manchester, etc) will think you are not a serious pilot. You have to file for a decisive entry into CAS.
The exception to the above is if you are departing from an airport in CAS (say Bournemouth) where a lower level filing should result in a continuous IFR service - providing the departure route is in CAS (e.g. the airway going to ORTAC).
I have just started up
FlightPlanPro and EGTC-EGTG FL100 gives me this awfully complicated route
-EGTC1000
-N0150F100 DCT BZN DCT
-EGTG0046
which validates for today at 1000Z. In fact that is the route offered by the new Eurocontrol "route suggest" feature, which is available via FPP if you use the Avbrief validation option.
The same route is generated for FL080.
For FL060 I get
-EGTC1000
-N0150F060 DCT NEDEX DCT
-EGTG0046
but as I say above that may be a dodgy thing to try and file.
The actual route flown on the say is likely to be significantly different from what you file, due to traffic, and this is true anywhere near the LTMA.
IMHO, to learn about real IFR, do a long flight somewhere e.g. EGTC-LKPR. You will get a few hours of practice talking to ATC and asking for shortcuts, etc.
I tend to find that the biggest bit of work is managing a flight where bad weather needs to be avoided. This is why I file high; one can nearly always ask for a "stop climb" if one finds that at say FL100 one is VMC with 200nm vis
Real IFR needs oxygen to be practical, due to the vertical extents of typical IMC formation in N Europe - unless one is so picky about weather that one's currency will end up being poor.
I think I sent you an email.