International models, and UK policy work to date, point towards a ‘deemed disposal’ on the day someone ceases to be UK resident. The gain subject to tax would be the difference between the asset’s value on that day and its base cost, potentially with rebasing if you became a UK resident part-way through life.
One possibility is that new arrivals to the UK would receive a ‘rebasing on arrival’, so that gains that arose before they moved to the UK are excluded from any exit tax. That structure was recommended in detailed modelling by CenTax and is widely used in other countries.
