This, from the interweb (source not stated, sadly):
""In 1891 a proclamation was made that members of the general public could hand in any gold coins that were underweight and have them replaced by full-weight coins. Any gold coin struck before 1837 also ceased to be legal tender."
so it would seem that Guinea may have ceased to be legal tender in 1891 as they were made from gold and struck before 1837."
However, this from Wikipedia:
"The Coinage Act 1889 also authorised the Bank of England to redeem worn gold coins from before Victoria's reign but on 22 November 1890, all gold coins from before her reign were called in by Royal Proclamation and demonetised effective 28 February 1891.["