I take your story entirely at face value. Sadly, the fact that you have kept hard cash outside circulation for this length of time may mean that in the UK it is difficult to put it into circulation. As murraymints alludes to, the banks have been co-opted into the police and must be/create a barrier to the use of anything but quite small amounts of cash.
To give you an example of the absurd lengths the obstruction goes to, when we (in a single transfer from my wife's account) gave our son £2000 to help him with a house purchase, a condition for proceeding with the purchase was placed whereby he had to prove where/how the money had reached him. In addition to providing a written statement of gifting, my wife had to go to a notary with our son's birth certificate to obtain acceptable written proof that she is his mother. The absurdity is that, while there is this sort of nitpicking intrusion into family affairs, annually literally billions of Pounds are being processed/laundered tax free through the London financial system and this is widely known and openly discussed. This is in fact effectively licensed by the UK government through the sale of "non-dom" status (I think I saw somewhere it costs about £35,000 - peanuts). The point is that such large sums keep the system afloat so it suits the UK to be so two-faced. The financial system is always quoted as being what keeps the UK afloat - the EU complains about the laundering and wants it to stop, is that maybe why Brexit suits the government ? That is another quite separate discussion.
My recommendation is that you choose a good lawyer and go and explain all of your story with the aim that he guides/helps you all the way through to using the cash. Then when the time comes it seems likely a lot of the cash you have will be in the form of outdated banknotes, a separate problem in itself.