Speak to your GP or health board and see which optician they recommend. There will be a list.
I get annual eye checks due to diabetes. They do various things, including photos of the retina. It’s not pleasant, but not unbearable. Just don’t drive there.
While most opticians may have the equipment and ability to do these tests, in my area at least only some of them are on the NHS list. As well as the optician doing the test, they send copies of the retinal photos to the diabetes clinic to allow them to be assessed by a specialist, and to allow comparison for future years.
Not all of the opticians are part of this scheme - there are around half a dozen opticians in my town, including a large Specsavers, but only 2 of the smaller ones do the diabetes tests.
It also seems to be regional, at least here in Scotland. The large Boots Opticians in a neighbouring town is part of the scheme, but I once tried to make an appointment at a Boots Opticians nearer my work, to be told that they could do the tests for people in the Glasgow health board area, but since I stayed in Ayrshire I’d need to go to an Ayrshire branch for the data to be transferred to the Ayrshire health board.