Afraid you've got a fake coin there.
Interestingly enough, I was able to find it in the Exonumia subsection of Numista: N#356146
As such, it must've been an incredibly common coin to forge.
The 1996 version of this coin does exist as a standard circulating coin (N#1390) as well as the silver proof & silver piedfort which it sounds like you've already come across.
COPIED FROM A PREVIOUS POST OF MINE:
The old round pound coins were heavily counterfeited. This was one of the major driving forces behind the switch to the new 12-sided coins.
The Royal Mint stated, in May 2015, that around 2.55% - roughly one in 40 - of all the round pounds in circulation were fake. In May 2014, they had stated that the rate of counterfeits was 3.03% - roughly one in 33. I can't find the exact original survey article (Google links to the modern page speaking about counterfeits of the new 12-sided pound), though there are plenty of newspaper headlines from around that era repeating the statistics.
The Numista catalogue is complete for UK round pounds. You can see the official list directly on the Royal Mint, which is reflected on Numista. https://www.royalmint.com/discover/uk-coins/coin-design-and-specifications/one-pound-coin/
If you have any mis-matched years & designs - they're fakes.