Andrew,
Send them a cheque with a long wordy letter stating somewhere in the middle of it that "... this cheque, if cashed, is subject to a 30 GBP payment processing fee payable to andrew by return. If TNT does not wish to be subject to this fee then they should send back enclosed cheque".
you'll be surprised by the number of people working in these places will just see the $$$'s and cash it without reading the letter. They will then be legally bound to pay you 30 GBP. If they don't pay it within 30 days then go to this website
www.moneyclaim.gov.uk/ and raise a claim for the 30 GBP plus "your costs" whatever they may be
be sure to attach the particulars of the letter you sent with the cheque.
You will find that TNT, being faced with a county court judgment (or CCJ) will pay the money plus your costs.
if they don't pay within 30 days you can instruct a government bailiff to collect the money on your behalf.
I know it's only 30 Quid but hey, it's the principal here.
FYI Here in Australia you can import anything to the value $1000 AUD without having to paying any tax