Thanks for getting back to me. It certainly sounds like you have followed up on Tina's symptoms very thoroughly with your local vet and this is great to hear. The combination of symptoms she has had certainly is a worry, and it is a concern that your vets cannot find a definitive diagnosis at this time. The symptoms you have mentioned certainly do not tie in with a severe gastrotintestinal worm infestation, and if you have been giving an appropriate dewormer (such as Milbemax of Drontal), then we can safely assume that worms are not contributing to the issue here. If you wanted absolute certainty that this is the case however, then it is just a matter of getting a fresh faecal sample to your local vet to have a faecal float run to rule intestinal parasites in or out.
So, assuming intestinal worms aren't an issue here - what are the next best steps? Your girl isn't young, but that isn't a reason to give up looking for the diagnosis here. Because this is certainly quite a complex case, I would definitely suggest that you ask for a referral to a veterinary internal medicine specialist. Thankfully, there are quite a number of these vets scattered throughout the UK, so if this idea appeals, please let me know and I can give you and idea of your closest veterinary referral centre. Here the specialist team can look at your girl's history, the work and diagnostics your vets have done so far, and from there carry out extra diagnostics as necessary in order to try and make a definitive diagnosis of her problem. I feel that any non-invasive, non-stressful diagnostic procedure that may help give us an indication of her issue, would be warranted here. This could certainly involve a cardiac ultrasound, and potentially even chest xrays, if Tina could tolerate these being done consciously. You would have to make a call about whether diagnostics that would involve anaesthesia would be warranted here. My thoughts are that your girl is too unwell, and too old to warrant the use of anaesthesia just for diagnostic tests.
Tina's case is certainly complex, and because your vets sound as though they may be a little stumped at this time, I would definitely recommend referral. Please let me know if you want help finding a local referral centre.
All the best with your girl and please keep me posted on your progress with the next steps.