Very generally, France is E from FL065 to FL115 (on the routes relevant to Eurocontrol filed flights), D from FL115 to FL195, and A above that. I am looking at the SIA chart now and that is the general idea.
You can file seamless Eurocontrol routes in France anywhere above (generally) FL065. They will be in E below FL070-FL110, in D FL120-FL190, in A FL200+. But all this is CAS for IFR.
(One can fly, on the day, in France, in Class G, of which there is plenty, by asking for various DCTs, but then ATC warns you that you are leaving CAS and asks if will you accept that).
However, it seems that someone in France has decided that D/A (FL120+) is handled by a different authority (generally Paris Control), from the lower level traffic.
And it appears that Paris Control (not the lower level controllers, however) has a LOA with London Control.
So, take the case of a flight from Dieppe to SFD (you have to see the map for this).
If you are at
FL070 at the UK boundary, you will enter the UK in Class G (Class A base is FL075). You will get handed to London Information. Your IFR clearance is gone. One could argue that the pilot should have used the VFR chart but not many foreigners flying IFR will carry VFR charts on a purely IFR flight, and a FL070 routing will pass Eurocontrol validation.
If you are at
FL080 at the UK boundary, you would enter the UK in Class A, and the obvious expectation is a continuous IFR clearance i.e. a handover to London Control. Actually you will get a handover to London Information. This will happen well before you get to the boundary - presumably because the French know very well that if they leave it too long they will be "responsible" for the inevitable CAS bust
A UK pilot who knows the ropes will now realise he must immediately descend below FL075 and continue "UK style VFR/IFR".
The above situation will also happen up to FL110.
Only if you are at/above FL120 really soon (i.e. climbing like a rocket out of Dieppe) will Dieppe hand you over to Paris Control.
Of course, flying from Dieppe back to say Lydd is hardly relevant - a trivial flight. The tricky situation is if someone is flying from France, in VMC but above icing conditions, at FL110, and he is transferred to London Information, which then orders him back down below CAS (base FL075). Unless he is a fast worker, and gets London Info to rapidly negotiate a new IFR clearance, he will soon end up at 2400ft, hacking around the LTMA
If he leaves it for more than about 30 mins (so I am told by UK ATCOs) then his original IFR flight plan is binned and he will have a lot of trouble climbing back into UK CAS.
The simple solution, as I wrote way back, is to climb from FL110 to FL120 and then you are talking to Paris Control and you get a nice handover to London Control
Whether your filed route is on published airways is nothing to do with this; most practical routes going back to the UK in the lower airways are off the published airways anyway.