Above 10,000ft in the UK and you are in Class A airspace and can't legally be there. <NOTE TO SELF: just check that before pressing submit>
Like many I assumed similar regualtions applies on the continent outside specialst areas.
If you don't show up on radar, you don't carry transponders and your flight path cannot be filed or predicted then I believe the onus is on you to avoid scheduled IFR traffic by whatever means possible (RAS/RIS/Airmanship).
Cheers
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