According to Kitco.com, gold’s price per kilo, as of today (i.e 20/10/21), is approximately equal to $57,434.
Its price per tonne is, therefore: $57,434 x 1,000 = $57,434,000.
According to Trading Economics, the UK holds 310 tonnes of gold in total (latest figure was reported as of June 2021).
Therefore, the UK currently holds: 57,434,000 x 310 = $17,804,540,000 of gold in its reserves (i.e $17.8 bn)