There is no tax/tariff/fee for importing an item valued at less than £18 (that's 18 GBP) from outside the EEC into the UK by mail, so a parcel declared as under $26 is (at current exchange rates) fine! So the Teensy is perfect.
There is no duty for importing electronic components, valued at less than £100+x (sorry can't find the regulation for x, but it's less than £50 from memory) from outside the EEC into the UK by mail, so a parcel of electronic components over £18, and under £100+x will only attract VAT.
As soon as a parcel from outside the EEC comes through the Post Office, and is declared as a value over £18, the Post Office will charge a fee of £8 to collect the tax and duty on behalf of the 'tax man'.
Every parcel I've received for several years now, valued at $30 or more has been charged £8 + 17.5% VAT on the value declared on the parcel.
So I'd end up paying £20 for the parcel and postage + £3.50 VAT + £8 for the post office fee for collecting the £3.50 VAT and giving it to the 'tax man'.
I always try to explain this to foreign companies before placing an order.
HTH