As Zipp Kits is an american company my guess is that by the time you have paid carriage and VAT the price will not look so reasonable and there will be a long delay even if they are willing to ship to the UK.
The Zipp pusher tug is just a Springer tug regading which there is pretty well unlimited information and free plans. You could try building one from scratch.
This thread will tell you a whole lot and includes the plans at post #4.
RC Groups Springer Thread