You don't have to wait to April if you still have any/all of this tax year's cash ISA allowance to play with. You can make contributions of up to £3000 in total into a cash ISA in any given tax year at any time. You can also stop/start/close/take out your money when you want - no conditions that way (assuming you take out an instant access one i.e. no fixed term).

The only real differences versus a standard bank account are:

No more than £3000 in total can be paid in in any one tax year

You get your interest tax-free

Hope this helps!