Just changed from BT to Vodafone.
I am using same Cisco 887 VA. I am getting a similar sync speed:
Attainable Rate: 76456 kbits/s 22851 kbits/s
But different download speeds:
Knowing Vodafone it's more likely to be an issue of capacity, namely that they might not be supplying enough of it due to sharing between too many users. This may be supported by the fact that your underlying line stats (attenuation, noise margin etc.) probably haven't changed much.