The only single platter drives for that sort of money will be either the 160Gb Hitachi 7K160 or the WD AAJS, I think the 160Gb 7200.10 is a single platter but I'm not 100% sure. There is a single platter version of the 250Gb 7200.10 but it's only just been released so I'm not sure whether it's in the dealer channels yet.
For that sort of money you could go up to 320Gb quite easily, the WD AAKS for example is only £47 + delivery. While this drive has two platters they're still the same 166Gb platters as the AAJS so there should be no difference in sustained transfer rates.