Here's my take on it: the mintage is low, but not nearly low enough to be rare - after all, the 1952 sixpence (a much lower mintage) is plentiful in lower grades compared to the 1981 currency 10p. I think (and this is simply educated guesswork) that the majority of the mintage was never issued, and quite probably melted down. Unusual? Yes certainly, but look at the context: no more large currency 10p's were ever issued. From 1982 to 1991 they only occur in BU sets, and then the small 10p arrived. So in 1981 - the last year of large currency 10p's - the Mint may have overestimated the demand for more coins.
Yet there hasn't been any media coverage of the 1981 10p, and even on eBay the sellers of generic 10 pences from circulation usually say 'pick your date' and then have a drop down menu that runs from 1968 to 1981 - I've never seen any of those have any stock dated 1981. So they must be pitifully hard to find in any grade?
(Having said that, there's one here: https://www.ebay.co.uk/itm/1981-Ten-Pence-Coin-Unused/373496292855?hash=item56f61d25f7:g:RvQAAOSw~kRgTMBg but I SUSPECT it's probably from the proof set?)