It's up to each individual CCI to look at someone's experience (logbook) and decide where to fit them into the "system". The medical requirements for over 40s don't apply to visiting jumpers as long as they fulfil the requirements in their home country. Obviously a low time jumper with less than 100 jumps is going to attract a lot more scrutiny than a very experienced jumper with "dues paid". In any event, you will need temporary BPA membership but, if you're staying for an extended period, you will need full BPA membership, currently 162 GBP.
You won't have a problem at my Centre but I can't speak for any others.