These days the best deals on Broadband tend to be for new customers, and so it pays to move every 12/18 months. However I see lots of issues when people move from LLU ISP such as Sky and TT, so when BT stopped paying for my broadband after I left them, I tried PlusNet, it has a good helpful forum and did what it said it would. I was on their unique 40/20 package, but at renewal time, although the rate was good, the CS peeps could not alter their script which included a bit about 38/1.9 which I did not want to accept. Instead I choose to stay with an BTW supplied ISP and went to EE - not least cos they had a good deal at the time, and added 5Gb data on my mobile phone package.
Where will I go next time, another BTW supplied ISP, maybe BT for their 55/10 package, or back to PlusNet if they have come to their senses and stop using the 38/1.9 product.