You are trickling info and you seem bemused by the responses thus far, your opening post gave limited info for us to go on so it would be expected of members including myself to question the set-up.
Your post 4 suggests you have not put front end protection on the tails with respect to your opening post as you suggest it as a solution.
What confuses me and worries me is in post 9 - you suggest fitting a 80amp fuse in the meter box - are your talking about altering and accessing the DNO's property here or are we discussing adding a switch/fuse in the meter box?
What you are doing requires front end protection, and standard cable calc's with respect to the installation methods and any expected insulation that may be added at a future date should be included if relevent, without you forwarding this information in your original query, we cannot give you any definitive answer.
With the required loft insulation requirements for buildings new or old, at some point I expect the cables will pass through insulation and require derating unless you have taken measures to avoid this, like I mentioned, we are lacking info to give you the answers you seek.
