Yes, as long as all the shares/investments are held within the ISA account you can do whatever you want, buy and sell all day long. But as soon as you withdraw money from the ISA account (to your personal account) that money cannot be replaced.
You can also have (I think) more than 1 ISA account. For example, you might have £20k with TD Waterhouse (the money has been built up over the years) and another account with Barclays. But you cannot deposit more than the yearly allowance, now around £10k for stocks.
Plenty more information online so get searching.