Whether it's illegal or not, the fuel duty in Wales (or for that matter the rest of the United Kingdom) is higher on AVGAS than MOGAS, so I can't see the powers that be getting overly irate about it.
100LL actually contains about 3 times the lead of the old BS4040 4-star, so you're more likely to have a problem with too much lead, than too little.
My suggestion would be to run your vehicle with maybe a 1/4 full tank, initially with a mix of 4 parts BS EN228 unleaded MOGAS and 1 part AVGAS 100LL, see how that works, if no adverse effects try 3:2 or 7:3 mix, and iteratively find a mix that works - which might prove to be 100% AVGAS 100LL of course.
The other thing to consider is using AVAS UL91, which is alcohol free, and lead free, but probably much closer to the fuel that old vehicles were designed around than EN228 or 100LL. If your main issue is with synthetic materials being degraded by ethanol, rather than lack of lead, that may be the better choice.