123.45
From the United Kingdom Aeronautical Information Circular
"ICAO Annex 10 Volume V Para 4.1.3.2.1 states that 123.45 Mhz shall be designated for use as an Air-to-Air communications channel to enable aircraft engaged in flights over remote and oceanic areas, out of range of VHF ground stations, to exchange necessary operational information and to facilitate the resolution of operational problems.
Within the UK and Europe there are a number of VHF ground stations operating air traffic and operational control services on 123.45 MHz. There have been numerous reports of aircraft within the UK FIR using this frequency as an unauthorised Air-to-Air open communications channel. Attention is brought to the fact that the resulting interference from unauthorised use of this channel is potentially detrimental to flight safety."
I suspect that the SE of England would not really qualify as being remote and a trip across the Solent to Bembridge is far from being an oceanic crossing.