An Individual Savings Account (ISA) can provide you with an ongoing savings plan, because you save not only in the tax year when you enter the money into the account but for as long as you want afterwards too. The ISA Future Value Calculator can help you to work out the total returns on your ISA savings (or investments, in the case of a Stocks and Shares ISA), if you keep your money in the ISA 'wrapper' over a number of years. As a caveat, you can remove money from the wrapper in an easy access ISA, but you cannot then put that money back in again.
In the context of your current year's allowance (10,200 GBP in total, of which 5,100 GBP can be put into a cash ISA), if you put in 4,000 GBP and then withdraw 500 GBP, the maximum you can enter into your cash ISA for that year is still only 1,100 GBP from then on. In the context of a previous year's ISA, if you have used up your full cash ISA allowance and then decide to remove half of that money from the ISA wrapper, you could not then top up that old ISA account at a later date. The ISA Future Value Calculator is a great tool for working out the advantages of using an ISA rather than a savings account to store your hard-earned cash for a rainy day!