Firstly, is it enforceable.
Secondly, if it were, would I owe £10 in Bitcoin now, or the amount of Bitcoin at the date £10 would get you 5 years ago?
I believe I've answered the first question, since:
Under the donative-promise principle, a simple, un-relied upon donative promise is unenforceable.
However, say hypothetically it was enforceable, would I owe £10 or £1000 (since bitcoin has seen a massive increase in 5 years)?
P.S. I never bought or owned the promised Bitcoin in the first place