That is a way confusing post, lets hope you end up with British Gas, it seems to be half of the cost of E-On.
How are you going to run your lights? 1 600 watt for veg, 3 for flower? Just as a reference for price.
From my initial review and a little math (i will show you how i got here if you would like) That with 1 veg 3 flower lights (18/6 for veg and 12/12 flower), you will be running about 85 pounds a month for the lighting part. You still have to account for: cooling, fans, actual ballast power loss, your normal household usage, and the service charges of having the service.
Based on their "average" monthly bill you will be a fair amount higher, at least 50% probably closer to 100%. So that very well may be enough to raise a red flag.
Here in the states we spend stupid amounts of money on things that use a stupids amount of electricity, ie hot tubs, tanning beds, salt water fish tanks, i could keep going, i don't know how common this type of stuff is in the UK, but could easily cost that much extra a month.