How do I change iTunes country with family sharing and child accounts?
original poster here - I did basically what Mitchell D suggested and it worked.   I actually moved my wife's account to UK first, set up family sharing with her, invited my child to join my wife's new family, then disabled my family sharing and moved myself to the UK, then invited child back to my family.  Phew!

So far, all seems to have gone OK but was a bit of hassle and entirely non-obvious!