If you are going to fly on the airways then unless you have spoken to the appropriate ACC / the Help desk, you have to stick to the RAD / SRS.
While you may fool the computer into processing your plan, the Sector(s) or ACCs affected may not be happy at all and could file an MOR. You could by fooling the computer for example end up departing without a CTOT when one was required. O you could end up being (not the first by a long shot) turned back at the FIR boundary.
Thus within the airways, fooling the computer into accepting your plan can result in;
a) No problem, ATC accomodate your unusual request.
b) ATC simply re-route you via the correct route and that is that
c) ATC report you for failing to comply with the requirements for operating IFR in controlled airspace.
No one can tell what will happen exactly but added to the above, how do you plan your fuel, diversions, weather and notam briefings if are not sure where you will end up?
There can be a big difference between the standard route and one you would like to fly eg entering the UK via Drake might suit you butit is against the SRS in general and you could be diverted by French ATC east of the Paris TMA to enter the UK via Traca. Quite a big diversion in a light single aircraft.
Far better to file standard routes and ask for short-cuts enroute.
Of course in France for example one can go VFR if VMC at the lower levels can be maintained and go as direct as one likes.
If I was flying from say Exeter to Lille, I would not follw the SRS unless I had to. Far better to get VMC on top and go more direct under VFR and if necessary back to IFR for the descent and landing. Making sure that the IFR/VFR and VFR/IFR change points are specified in the FPL.
Regards,
DFC