Lewton is on the ball.
There is no official frequency allocated for air to air chat in the UK.
123.45 is allocated to several airfields, at least one of which has a published instrument approach procedure.
It is very poor airmanship, but not uncommon amongst some G/A pilots, to just pitch up on a frequency and chatter between yourselves, at the very least you block out someone who is legitamately using the frequency - you could also be blocking a more important call.
If you are flying with another aircraft, the best thing to do is call an atc unit, advise you are flying in formation, and if traffic permits, ask for permission to talk through to your colleague/s. However, use it sparingly.