Basically, lots of work places/services/insurance etc seem to be under the impression that GPs provide letters confirming medical conditions, and insist that you have to have them before they believe you.
But my GP at least will never write such a letter. I had this issue two years ago (hospital consultant eventually wrote me a letter) need one now to prove to an overseas company that I am pregnant, but the response from GPs secretary "well we can't because you'd have to pay..." Me: "No problem". Them: "well actually no we don't do that... try the midwife...."
Except the midwife is not a Doctor and a letter from them won't be accepted.
I realise the NHS is overstretched and don't really need this kind of paperwork, but we can't really change the way the whole world works in requiring medical letters, and it's ridiculous that I can't pay the Dr to look at my medical record (heavily pregnant, recent ultrasounds etc) and go 'yep, pregnant'.