Thank you for the replies. I will send pm.
ThisNickIsReallyFree I know you have to declare if you are carrying more than Euro10,000 in cash when leaving the UK (you are allowed to but need proof that it is yours). I am not aware of any rules about taking coins and it isn't something the UK would be concerned about. The only government information I found is here:
https://www.gov.uk/bringing-cash-into-uk
The one exception to this is treasure. There are special rules in the UK about treasure - so if you dig a hole and find Roman gold coins there are rules that apply then.
Maybe others can comment if there are any export rules but I cannot imagine (other than treasure) there are.