Capital Gains on sale freehold & shop premises

My dad who is in his 90s has finally decided to sell a shop which he has owned since 1971 and which has provided an income to him for many years. I'm trying to organise the sale on his behalf to the current tenant and want to minimise the amount of CGT he has to pay. I've read a little about this on the gov.uk website.

I think he has to pay 10% CGT on the increase in value and I think we have to value the property as at 1st March 1982 as it was purchased before 1981. Not sure how I would go about doing that.

Dad's paperwork is non existent so proving how much work and improvements he's made to the building over the years is going to be difficult.

Has anyone got any advice about how best to go about this? Any pitfalls you're aware of? Any tips? Many thanks