How much so you want to invest? Lump sum and/or regular contributions? Do you want a guaranteed return or are you prepared to take a risk? Returns are very low.
As factor has pointed out, currently the returns on savings are pretty poor right now.
My folks recently put just over 5K into the Halifax - return is as poor as every other institution, but you do get the bonus and fun of being entered into a draw held every month, where 3 prizes of £100,000 are awarded.....
Don't all ISAs return capital, with varying rates of tax free interest? As Factor says, at 2 per cent interest, that would amount to £100 pa. So next year your £5,000 would be £5,100. Every little helps,so go for it.