Endometriosis is plausible - mention the symptoms to your GP as being monthly and ask for a referral, or try the pill to see if it helps. Could be part of your IBS too - IBS symptoms can be worse with periods, and can extend to the GU system. Unlikely to be MS etc if the bladder empties normally.
Btw, i get you're pissed. But with the number of referrals and expensive investigations you've had you can't say that they aren't 'taking you seriously'. And what can they do if they have tried medication that you refused, and there is nothing to find on any investigation?
I doubt it is a 'deep seated infection in the bladder' - that seems like crazy talk and there would have been some evidence of it in blood tests or cystoscopy or urine sample.
Maybe, since you're pissed with the NHS, you should just take the plunge and go private. I know you said you don't want to, but sometimes if your convictions are that strong despite the evidence that you're not being taken seriously and having enough investigations you have to risk being 'ripped off' by someone who will just do whatever investigations you want to be done. You never know, you might even be right.