Now another question arises.
Apparently there are people collecting banknotes with funny serial numbers. Those could go for a lot on the collectibles market.
What if you bought, say, a £50 note with serial number AA01 123456 for £1500, and reward it to your employee and declare to HMRC as £50?
I think it's perfectly valid you may have accidently come across such bank note and unknowingly paid it to your workers, who later realised he could profit on the serial number. It's also possible that you deliberately used collectible bank notes to trick the tax authorities.
And extending that hypothesis, you could reward employee with a sovereign with £400 declared value, which is about the market rate "to the best of your knowledge" but a 1917 London, which then could be flipped for £20,000.
My guess is whether your intention was to pay your workers a bonus in ways alternative to a bank transfer, or to make a covert payment of a large sum. In this case, it might be down to whether you have told the recipient that they could later sell the note/sovereign. It could also be whether you have made recent purchases of collectible items (which then implies the intention of using them for payment)
Any legal experts?