My ISA provider pays interest at the end of the tax year. If I withdraw my money as cash before the end of the tax year (e.g. to use as a deposit), will I lose the interest accrued up until that point?

It depends on the T+C’s of your product – could be either depending on what you signed up to.

There’s a difference between accrual dates and payment dates. Your initial paperwork should tell you (probably says interest is calculated on a daily basis but is compounded annually).

Phone them up and ask.

Okey doke, will check it out. Thanks.