It is not possible to provide a consistent series for the official reserves before 1999, given changes in accounting methodology. Back data for the UK's official reserves prior to 1999 can be found on the ONS publication, “Economic Trends Annual Supplement No. 32 2006 edition.”
http://www.statistics.gov.uk/downloads/theme_economy/ETSupp2006.pdf
Detailed historical data for the official reserves on a consistent basis from July 1999 can be found on the Bank of England website:
http://www.bankofengland.co.uk/statistics/reserves/index.htm
This information can be found at:
http://www.bankofengland.co.uk/statistics/reserves/Tempoutput.pdf