If your health is in any way suspect get a medical first. Too many folk die in the UK when stress gets the better of them underwater. Lack of dive and general fitness even among regular divers gets a number of folk around April in the "silly season"
I would also say that if you are diving with anyone, and most organisations recommend buddy diving, that if you have a problem underwater the odds are that you have a good chance of taking your buddy with you. Folk understandaby take a poor view of this possibility.
Diving isn't rocket science but it is not an armchair sport either, there is a fair amount of physical effort involved diving in the UK simply caused by conditions and the kit required. the video of a recent trip shows how much kit can be required (although you can get away with considerably less the drysuit and weightbelt and de rigeur)
PFO's, if that what you have, can be cured in a lot of cases and a large percentage of the population have one to a greater of lesser degree. The disavantage of having one is that you are also at a greater risk of decompression sickness and believe me you don't want this either because of the immediate effects and issues or the possibility of permanent damage.
I am speaking through personal experience of myself and others that ghiding a medical condition will just catch up with you and clubs and trainers will take a very dim view, to say nothing of any qualifications probably being invalidated.
The short answer is don't do it in the UK unless you have a medical and be careful abroad where at least the effort required in holiday resorts is likely to be considerably less and more tolerable.