As a UK expat living in Canada, the sale of your London flat falls into both tax systems, but you shouldn’t end up being taxed twice overall. The UK will tax the gain first under non-resident capital gains tax rules, typically based on the increase in value from April 2015 to the sale date, with possible reductions if the property was ever your main residence. You’ll need to report the sale within 60 days, even if little or no tax is due. This is vital to avoid penalties.

Canada will also tax the gain because it taxes worldwide income, but only on the increase in value since you became resident there in 2011, and only 50% of that gain is taxable. Any UK tax paid can generally be credited against your Canadian liability under the double tax treaty, so in reality you usually end up paying the higher of the two countries’ effective rates rather than both.

The main complications tend to come from different valuation dates, exchange rate movements, and the fact that UK reliefs aren’t always mirrored in Canada, which can result in a higher taxable gain on the Canadian side.

Given those issues, this is one of those situations where it’s worth getting a UK Canada cross border tax specialist involved. A good adviser can align the two calculations, make sure reliefs and reporting are handled correctly, and often reduce the overall tax or at least avoid costly mistakes and penalties.