A Part 21 aircraft is not an issue and can be flown anywhere in the world subject to ICAO rules. The main issue is likely to be your medical. If you are flying on a sub ICAO UK LAPL/ self declaration medical then that may be an issue. In the past France have accepted NPPL holders as you say but the pilots were required to hold an ICAO recognised medical such as the UK class 2 as a minimum.
The first step is to write to DCAG and ask them for their acceptable requirements and the process.