Beds and mattresses can be as cheap or as expensive as you like. Personally I would say to spend on the bed in line with the rest of the furniture in your house or room. That way you're probably spending the amount you want to.
The Genoa was mentioned above so do you think this one will be ok, ie, genuine like the one at Dreams?
http://www.bedroomworld.co.uk/p/Genoa_Ottoman_Bedstead.htm
This is the same bed at much more money at Dreams:
http://www.dreams.co.uk/genoa-bedstead-brown/
Also, can you choose me a firm mattress or perhaps medium firm from this lot that would be good? I'm wondering if any of the sub £200 ones would be good? More cm depth is preferable, ie, over 18cm.
http://www.mattressman.co.uk/mattresses/double/