Another thought I just realised that you are UK based and should be experiencing 3g speeds. But, a review of your phone says:

The phone packs EDGE and WCDMA (UMTS) 3G on the 2100MHz band with fallback to GPRS when those higher speeds standards aren't available.

Is it possible that because of a configuration setting or glitch, the phone is falling back to GPRS when data is being sent through bluetooth?