both eyes are open but the left eye's gone blurred. Ideally, I'd get it checked out first before taking an eye test.
You certainly should because that's not normal.
Vision will blur with age. Presbyopia. But that's a process that occurs gradually, over a period of years. It also doesn't only affect one eye.
You don't need an optician. You need an opthamologist, a doctor specialising in eyes. Preferably now. While a standard eye test will pick up quite a few eye-related medical problems it shouldn't be a substitute for an examination by an opthamologist or at least a doctor.
It's not the test, it's the cost I'm more worried about. What I really wanted to do was have it checked out first bu65t I was just told to take an eye test. I'm not on benefits so can't have it done on th NHS so it'll have to be done private. It's the cost of the glasses I'm worried about
I doubt you'll need glasses. I expect you'll need medical treatment.
Cost of glasses is between £20 and £500 for functionality. Any amount for fashion. Most people will be fine with a £20 pair made by a glasses maker. That's the cost when "done private". You need an optician for the test (which will be £20-£30) but you don't need an optician for the glasses. Get the prescription, give the details to a glasses maker, glasses maker feeds it to their machine, job done. You'll probably have to ask the optician for your pupillary distance as they often don't include that in the prescription. Presumably to dissuade people from using glasses makers instead of paying a far higher price to buy glasses from the optician. They might charge an extra fee for the pupillary distance. I've no idea if that's supposed to be allowed or not, but it happens.
For a complex and/or more extreme prescription, an optician might be better. Glasses makers are certainly fine for common visual defects to common extents. I'll pay £300+ for a pair of glasses in the cheapest available frames, but that's because my natural eyesight is utter crap and if anything about the glasses is off by the merest fraction it's a problem.
But
sudden onset blurred vision is
not an optician thing. It's a doctor thing.