Another option could be one of the non-protected training frequencies for the appropriate aircraft category, i.e. Glider (when I was young we used 130.125) or Microlight (129.825) or even the shared Glider/Hangglider/Parachute (129.900). As far as I am aware of, there is no motor aircraft training frequency in the UK, or?
Expect a less than cordial response if you start yakking on any of those frequencies.
One of the better solutions is to use non-airband radios aka walkie-talkies. That way you can communicate with each other and monitor ATC at the same time plus you won’t annoy anyone else.