Good luck. They've been avoiding the requests since 2016 or so when the then CFO who was amiable to providing such info was no more. Up until 2015 the data was available.
"Although we hold information within the scope of your request we are withholding this information pursuant to section 43 of the Freedom of Information Act (FOIA).
Section 43 FOIA – Commercial interests
It is our view that the release of information in respect of mintage figures for bullion coins would be likely to prejudice the commercial interests of the Royal Mint as well as the market and end customers for these coins generally.
Core bullion products (e.g. Queens Beast, Sovereign and Britannia) do not have mintages, certainly not in the sense that we do with commemorative coins. Instead the production figures are solely determined by market/customer demand. Declaring production figures on these coins has the potential to impact sales and falsely inflate the prices in the bullion market. For example, if the production figure is considered to be low then the product could be viewed as a rare/collectible item and in these cases many retailers increase their prices. This causes confusion as to the distinction between core bullion products and commemorative/collectible items. There is then the clear potential to negatively impact the end customer as they could end up paying more for a bullion product than its intrinsic value. It could also impact negatively on our numismatic business as these products are sold in that team too. We note that our stance in this regard is just the same as with many other Mints (e.g. the Royal Canadian Mint), neither of whom declare mintage figures for bullion coins, but do for commemorative coins in accordance with their publication schemes."
Makes no sense but that's what they're likely to reply.