goggsiewoggsie

If your preferred position is that you end up with £0 in your ISA at end of the tax year then there's no particular reason to wait until then to do it. If you move the £15k into an account paying 0.2% extra now, for the ~133 days remaining of the tax year, you'll get about £11 more than if you left it were it is.

This assumes your total interest you receive in this 2017/18 tax year isn't going to be over your personal savings allowance or your other allowances (or any special tax band for low earners), and so there isn't any tax to pay on the interest income on the £15k you get between now and tax year end, even if you did earn it outside the ISA.

For example, if you had a salary of £150k then you wouldn't qualify to have any personal savings allowance and would be on a 45% marginal rate of tax on new earnings - so if you earned and received £69 of interest on the £15k between now and 5 April you'd have to pay tax at 45% which is £31. You would have been £20 better off just leaving the money in the ISA to earn £11 buy with less interest with no tax to pay.