In the 2019-20 tax year (a tax year runs from 6 April to the following 5 April), you can deposit up to £20,000 into a cash ISA. You are only allowed to open one new cash ISA per tax year, so if you opt for a 5-year fixed rate ISA, for example, you will not be able to open another cash ISA until the next tax year. The only exception to this is if a provider allows you to ‘share’ your ISA allowance across their range of cash ISAs.
You may only be able to make a single deposit and/or transfer in to your fixed rate ISA when you open the account. You might be permitted to make further deposits into the ISA, but this is only while the product remains on general sale. However, as soon as the fixed rate ISA is withdrawn from sale to new customers, you won’t be able to put any further money into it.
Although you will be able to transfer money out of your ISA, there will usually be a hefty interest penalty to pay. If you think you’ll need access to your money before the end of the term, look instead at an easy access ISA, one that offers penalty-free access after a notice period has been served, or fix for a shorter term.