Good Afternoon Caroline,
Shopify charges Canadian tax to Canadian customers. At this time we do not have any physical presence and are not billing tax in other locations. We do not charge tax to customers in the UK so the one factor will be exchange rate from US to GBP on your credit card. We bill in US dollars for all customers since it can convert to your local currency with almost every credit card.
If you are concerned about fluctuations and projection we do offer a 1 year (or more) plan (you can sign up at any time from the Settings>Account page of your admin) and we also provide an additional 10 percent discount for this yearly plan.
Feel free to get in touch via email or chat if you have any other questions while forming your plans and setting up. We're here to help answer anything we can along the way!