I recall a thread a few years ago similar to this. I put up a post then stating that the CAA would not prosecute for a visibility related infringement of the old rule 5. The gent who told me that has now long gone but hopefully the policy survives.
Do not crash to avoid prosecution. If you need to fly lower, slower, or land, do it. Dont let the law cloud (ha, ha) your judgement if up against it. Flying in cloud is really crap. Been there, done that, didn't like it.
EGNH (nearest report) TAFs in my experience are a guide only for Lancashire's hill country. If you fly long enough in Lancashire you WILL end up in weather totally different to the forecast. I have now done all of the following. Landed, turned round and gone home, gone up, gone down. In fact everything except crashing. Going up tends to make it worse. Favourite is any hint of poor vis do a 180 and go home. Oh and I'd rather not be in a Robbie either if it gets thick.
I dont yet have thousands of hours but have more than a thousand flights in Lancashire.
I would expect the same applies over quite a lot of the UK.