I think in general it's better to keep as many records as possible if you're wanting to keep "above board" so to speak. I don't think HMRC have an official stance on Bitcoin in regards to mining specifically, and I'm not sure how much of a response you would get if you contacted them. I agree that recording market rate at the time of mining would be a logistical nightmare and even setting up some form of automation would require a bit of work.
In the long run, the more records you have will set you in good stead if HMRC eventually come knocking. It's a matter of deciding how much detail you actually go into though, specifically when it comes to mining.