It is possible to get a restricted class one medical (and therefore train for and get a CPL in order to be paid for flying). A class one restricted medical may have its validity restricted to UK registered aircraft in UK airspace, so not a problem for instructing.
An example of a case for a restricted class one would be if you met the class one renewal standards, but not the class one initial issue standards, and could demonstrate that you had safely flown many hundreds of hours without incident. As far as I can tell, each case is carefully assessed on its merits.
If you wish to go this route, then my advice would be to join AOPA who have all the advice on how to proceed.