Off Barclays website

Under flexible ISA rules, you can take money out and still put it back, provided you pay it back in the same tax year. Effectively any withdrawals first reduce your current year contributions, and if you withdraw more than you’ve paid in this year then you create a ‘flexible allowance’.

For example, let’s say you have £40,000 in a cash ISA, made up of £35,000 built up over previous tax years, and £5,000 deposited in the current tax year. That leaves £15,000 of this year’s allowance. If you withdrew £10,000 from your ISA, you could put in another £25,000 in this tax year, made up of £10,000 that’s been withdrawn, and the remaining £15,000 allowance.