Probably illegal in the UK because one is not allowed to record ATC.
Although you can get kit in the aircraft to record ATC
- Garmin's G1000 (fully EASA certified for all its potential, and superb) will record the last few seconds of ATC chatter as a way of playing back clearances to make sure the readback is right. Great way of double checking if someone's read back a clearance without including their brain in the link!