I listened to a MoneyBox radio programme about this. The "financial expert" said that it was effectively a tax-deferral program, just like a pension plan. Any gains inside the fund were tax free until the money was brought back into the UK. It may seem unfair at face value, but any UK citizen can defer taxes on up to 100% of their annual income in a registered pension plan[0]. So, IMO, the fairness would depend on what investors can do with the offshore fund that regular people can't do with their pension plans?
0 - https://www.gov.uk/tax-on-your-private-pension/pension-tax-r...