Hi 5 <G>;
If the next controller indeed does issue a different altitude then the former restriction is indeed canceled. Can't speak for the UK rule book as I haven't seen it in a bit, but the ICAO agrees with the US book here. If you put a restriction on an aircraft and need it for separation and you don't pass that on to the next controller you can't expect it to be complied with. It has been that way for the last 26 years that I have been doing this...
regards