You've got the early part of this rather the wrong way round. In 1931, under Bretton Woods, the UK fixed it's exchange rate with the US dollar. At the same time, the US guaranteed to fix the price of gold at $35 per ounce. Therefore, sterling could be converted into (US) gold. Technically...
This has not been the case since the collapse of the Bretton Woods system in 1971; and the UK has not had a 'proper' gold standard (which is the term for having a currency supported by a hoard of gold) since 1931 (Bretton Woods tied sterling to the dollar, and the US at that time were willing to...