We bought a junior bed off ebay (mamas and papas, normally £100, got it for £25 ) for dd1.
I wouldn't have paid full price for one, but if you can pick up a good 2nd hand one, I think junior beds are great between ages 2 to 5/6. dd1 turns 5 in April and she will be too long for hers later this year I imagine, but with dd2 turning 2 in December, should be perfectly timed for us to buy dd1 a king size single bed (not sure what they are called in UK, we're in Australia), which should last her through high school hopefully. We thought it would be nice to give her a little bit of input into her big girl's room, when we do it up later in the year.
A friend had the Ikea one for both her children and she was happy with it. Gives you a few more years to get organised for space as well, if the child is in the smallest b'room, as well.