Perhaps if the UK SSR system was as pilot-friendly as the US system, this wouldn't matter! It seesm that every time you change frequency (certainly on military frequencies) you have to change the SSR code in the UK....
I was told that changing squawks without selecting 'standby' was fine as it takes 10-20 seconds after changing codes for the new code to be transmitted - or perhaps that really meant for the new code to be registered by the radar heads which were around 10-20 years ago?