I have an update!
Today I have received a letter about this. Not an invoice from Customs or a telegram arse kicking from the Queen but, as expected, from Parcel Force. They haven't tried to fine me (I can only assume because they realise they have absolutely no power to do so) but an invoice for £8. Funnily enough there is no phone number on it to talk to anyone about this fee, only a payment line. I will try to ring the line but I suspect it'll be automated, in which case I will send a letter to the payment address instead, I may also include a cheque payable to "The Robbing Bastards at Parcel Force" it's be worth the £8 if they tried to cash that.