I'm struggling to see where the issue is here??
"this is the best deal available from our insurance underwriters" could mean with the current scheme you would have been on. They are able to offer you a discount because they can then 'rebroke' and find an alternative insurer (or scheme) to place you with. This is a fairly common complaint within the MoneySupermarket forums and most think that the insurer/broker/intermediary should be the ones finding the best deal at each renewal. It's absolute rubbish and not feasible to do so. If the client is happy paying at the rate specified, then they will. If not, then they should call up to see if there is anything better they can do and offer a comparison price.
If the insurer had said "this is the best deal available to you on the entire UK insurance market" then you would have grounds to complain, but it does highlight that it is specifically the best deal from them whilst you are on the current scheme.
The only thing they are guilty of, is shooting themselves in the foot by not saying something like "Give us a call if you find this cheaper and we will match this" or something to that effect.
Kind Regards,
Vicissitude