There is no documented evidence of a minting flaw in the "Festival of Britain" Crown, but it is possible. A reputable coin dealer will be able to advise.
There was no "Festival of Britain" Florin minted, you possibly refer to the "Festival of Britain" Crown (Five Shillings). See "What is the value of a 1951 British Crown - Festival of Britain".
Do you mean the Festival of Britain in 1951?
in britten Britain in 1951.
1951
The Festival of Britain was a national exhibition which opened in London and around Britain in May 1951. The official opening was on May 3.
1951
1951
It depends on the date, denomination and what you call a mistake, post new question.
15 years is a very big mistake especially for a gold coin. I suggest that the coin is not what it appears to be. A reputable coin dealer will be able to identify your coin and give a valuation if it turns out to be genuine.
It was held in 1951
4000 gbp
El Paso, Texas