Advantages of opening an ISA account one week before 5th April

Yes, but only in a specific set of circumstances.

In each tax year (6th April to 5th April) the government allows you to put into an ISA the ISA allowance for the year plus any amount that you have taken out of ISAs in that year*.

• So, if you currently have (or can get hold of) this year's ISA allowance, you could open an ISA before the end of the tax year, and deposit the full limit.

• At the start of the next tax year, you could take that money out and spend it as you'd originally planned to (or repay wherever you got hold of it from).

• Later on in the next tax year you could deposit in an ISA the amount you'd withdrawn (i.e. this year's ISA allowance) plus the current year's ISA allowance

This obviously only makes sense if you expect to have, at some point next year, more than next year's ISA allowance to deposit. And you can only do it if you have access to the funds to park it in the account over the change in tax year. So, yeah, in only a very specific set of circumstances is there an advantage.

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*Note that although the government allows this, not all ISAs include this feature, or allow withdrawal without penalty. Check the Terms and Conditions of individual ISA products if you plan on following the approach in this answer.