If you click on the link that says "Secure CC: PayPal Secure Payments" (at the very top of the Tixati webpage) an new webpage opens up that allows you to make a donation via PayPal. That webpage clearly shows that the prefered donation is in Canadian Dollars. (Did this myself from the UK and PayPal handles the conversion from GB Pounds to Canadian Dollars for you.)
If you want to get some idea of what the donation is going to cost you then I'd recommend using a currency conversion website like
http://www.xe.com/currencyconverter/ The amount that it shows for your conversion won't be the same as PayPal uses but it will be close enough in the park to give you an good idea of what your donation will cost you in GB Pounds.
Good to see you want to donate to the project. Hope this helps.
