Hi, I live in Lympne and had the same issues as you. I've been with Callflow for 2 years now and the download speeds are excellent compared to what they were...my speeds were similar to yours. I now get a 32 meg download speed so can now stream movies / tv episodes without the dreaded buffering. I work from home and everything is fine now, no issues. The only time that I've had issues have been when the service has dropped completely due to a fault.
If you have SKY TV via a dish then Callflow does not interfere with that, if you get a discount from SKY due to having TV, phone and broadband then you'll lose that as your broadband will have to move to Callflow. I think they can do a broadband only so the phone line stays with SKY by I'm not 100% sure.
Another thing is with SKY you get SKYGO online service and this works really well with Callflow due to the good download speeds. I use it to watch the football on my laptop if the wife wants to watch a movie
Hope that helps.