So you can fly uncontrolled throughout Europe without talking to anyone.
That's news to me.
Each country has surveillance going on, for national security purposes.
In practice, it is not strictly operated, and in any case it is often not clear what form of contact should be used on the handover. Take the silly case of Lydd to Le Touquet. You cross the FIR boundary, switching from the Lydd Frequency direct to the LTQ frequency. Taking this further, you could fly Bournemouth to Rouen with just the one frequency change, and I am sure nobody would question it.
Flight plans are mandatory for any border crossing but again in practice one can get away with a flight even if the FP was lost (has happened to me many times; even IFR/Eurocontrol FPs get lost sometimes). ATC just deals with it.
But it is best to not push this. In a well-publicised stunt, one man flew a Vari-EZ UK to Switzerland non-radio and non-FP (having initially pretended to be doing a diversion within the UK) and got done £20000 by the UK CAA. There was some appeal later regarding the size of the fine but nobody seems to know the details.