There are no statutory requirements relating to the publication of pardons granted under the Royal Prerogative of Mercy. However, by convention, the Clerk of the Crown in Chancery places a notice of free pardons granted in England and Wales in the London Gazette.
In the past 20 years, only two free pardons were granted by Her Majesty the Queen in England in 2009 and 2014 respectively. I am not aware of any other pardons (by use of the Royal Prerogative of Mercy) that have been granted in the UK.
However, with regard to Remission Pardons for England & Wales no records are held for the period prior to 1 February 2013; since that time the Royal Prerogative of Mercy has not been exercised.