I'll add one possible end-user niche for the owner or his buyers to explore:
Fibonacci retracements are an important tool for day traders. You might be surprised how high the keyword competition is for that phrase.
So fibonacci.org.uk may be more valuable than some people on here realize. The word will have extra meaning for day traders.
(In fairness, I own some 2-word non-.CO.UK domains in this area, which is how I know that Fibonacci.org.uk has this application.)