Hi Phil,
There's a list of UK places that do the testing here:
https://www.s4me.info/threads/uk-locations-that-offer-cpet-vo2max-testing.4999/
There's also info about the protocol in that thread too. It's pretty decisive in testing for PEM, rather than other types of exertion intolerance (MS patients, for instance, report exertion intolerance but then don't get the drop in function on the second day that ME patients do). It will also show you if the drop is in VO2max, maximum heartrate, workload, etc. That might be a clue.
I suppose it's possible that if you're in recovery you won't get the clear drop in function on the second day, but it might still be worth a shot to put your mind at ease. Sometimes, when you get diagnosed with ME, doctors attribute everything to this, which is unhelpful if there's something very treatable happening in addition to the ME.
The heart pain and dyspnea might be clues--my heart hurts when I have an asthma attack, for example (and I can feel it palpitating). My asthma also gets worse when I pick up a virus, which can last for a while.
You've probably been tested for asthma already, and I know you've had an ECG, but the breathing + heart thing for me = something cardiovascular (though ME can affect cardiovascular health). So, as I say, it may be a clue.
POTS is the other thing to consider, as Ryan says, especially because, anecdotally, exercise may help POTS in some cases. The NASA lean test done over a long period of time (10 minutes+) can sometimes work and can be done at home or in your GP's surgery. But if it's a more complex type of dysautonomia, then it may require more complex testing. Dysautonomia is common in ME, so it doesn't necessarily mean it would replace that diagnosis, but may overlap. Especially if the two-day CPET is positive.
Good luck!