Warren Evans.
I just rang them up to get DS his first full-size mattress, because they're Which's recommendation. I've got his actual bed frame second hand from Ebay, because I am tight about stuff that isn't essential, but a mattress is important IMO, so I was looking at pocket sprung as a minimum, and potentially hypo-allergenic as he's asthmatic. They have this super-dupa all natural lambswool padded one that even in the sale is over £500, so I wanted their advice on whether I could use an anti-allergy cover instead, and go lower down their scale. The lovely girl I spoke to, Amber, immediately said Warren Evans don't recommend pocket springs for kids under about 12 and absolutely not toddlers, because they're not heavy enough to benefit from the support and in fact they get less support because they aren't cushioned by springs they can't depress. In the sale, the mattress she recommended was £135.
I can't think of many companies that reject a gullible parent's willingness to fork out an extra £400, solely in the best interests of the child. I'm certainly not going to be buying anywhere else in future, given the reason I looked at them to begin with is they were Which's top pick for comfort and durability. And when I poked around their website, they've won a ton of awards for green/ethical manufacture, if that's important to people (have to admit it primarily is to me because of my son's asthma). And one for UK manufacturing excellence.
Dreams were a Which don't buy, incidentally. A lot of unhappy customers with not very comfy, not durable mattresses, and a heavy sales pitch. Sealy and John Lewis were also said to be good. (Oh, and Vi Sprung came highest of all for comfort, but we would be remortgaging to afford one.)