Hi
Do NOT get an ECG. We have had numerous postings here about false positives on ECGs - ie a normal ECG but either the machine says heart attack or the leads are in the wrong place. It takes time and money to correct.
In fact there is some logic in all young people having a check ECG and many schools in the UK offer it at about 14 to rule out specific problems but it is very very rare to show a problem
You really would be best advised to leave everything until you do your Class 1 initial which will I hope be read by a human!!!
You will I am afraid need to declare your possible OSA, and you may need to be properly tested with a sleep study - the FAA are getting very hot on this, and I expect the UK will catch up. Ensuring you are not overweight is all you can do in the meantime.
Good luck