Yes, tell them about it - it's a 'condition'.
I didn't for decades, as even though it took me about 25 years to be diagnosed, even after that the medics still never explained the seriousness. It sounds mad, to me, now, but I had no idea it was a 'condition'. I also have a miniscule fibroid and was horrified a few years ago to find that this also counted as a 'condition'. A friend had apoplexy when I told her, as she had previously had bad fibroids, that caused incredibly heavy periods, but she had never declared them, as it took the medics so long to take it seriously, plus they just fobbed that off, too - for ages - as part of the 'just periods' mindset. For years after my endo diagnosis, I still thought in terms of having 'bad period pains', which is what medics had fobbed me off with for years, and no one ever said it was a 'condition'.
My more recent declarations have not affected my premium any further, although iti s already hiked up by another condition. If your endo is mild enough, and you have never felt you needed a GP, or were at risk of hospitalisation, you could ask to have it excluded - if they do want to charge you - but it does mean you would have to pay if you needed medical help for the endo, while on holiday. But you do have to declare it first, if you choose to have it excluded: plus... not all companies allow you to exclude things.
I personally would exclude mine, now, as it was always controlled by painkillers, or the pill, and now I'm post-menopause, yet still with a mirena (that stopped the pain for me completely), I'm still utterly pain-free.
But you have to weigh up the pros and cons, according to your own pain and treatment record. Why not talk to your GP about it? Mine gave me a printout with all the things listed that they class as worth mentioning - which was a shock.
If these companies were truly honest, they would make an effort to define 'pre-existing medical conditions' properly - but you have to remember they are just out to make a profit. The Ombudsman has pulled them up about this repeatedly, as TI companies are the bodies that the Ombudsman get the most complaints about, every year. However, the last time I checked, most had still not clearly defined 'pre-ex conds'.
I think we should all ask for everything in writing, maybe even record our conversations - although I've no idea if that is legal. Also, maybe we should start writing to the ombudsman every time we are unhappy with a service.
Not that I'm paranoid about TI, or anything!! I hope you get sorted!