AS others have said, they cannot force you to allow a medical report, but it is in your interests to do so.
Firstly, there is sick pay. You do not mention whether you are getting any, but often continued to receive sick pay above SSP is linked to co-operation with requests for medical reports. Look at it from your employer's point of view. Whilst they may be sympathetic (and I hope they are), there are, sadly, enough people who swing the lead that employers want to check people out. Therefore they will often link your sick pay to medical feedback. Once they get a nice clear report from your doctor, they can feel more confident that you are really ill (not suggesting in any way that you are faking it, just that some people do).
Secondly, as others have mentioned, there is the disability legislation. PND often won't be a disability, because the key characteristic is that it has to last, or be expected to last, a year .However, if your doctor does think that your symptoms might last this long, you get an extra raft of protection against dismissal.
Finally (and assuming that you have been with your employer a year) you have unfair dismissal rights. Employers CAN (contrary to popular belief) dismiss for ill health. It is what's called a capability dismissal. However, to do it, they need to be sure that you're indefinitely unwell. If your doctor is saying things like "She's not well enough at the moment, but, with the right treatment and support, she may well be in a month or two" it actually makes it HARDER for them to fire you.
All in all, it's in your interests to give your employer as much information as you can. It should make it more likely that you get fair treatment and less likely that they can dismiss you. It's not really about how much you've told them so far. It's about hearing it from a professional.
Hope that helps.
I would give consent, but explain to your employer that you want to see the report first (this is your legal right) and say so to your doctor too.