When I did the calculations recently, thinking of a switch, honestly the savings didn't come out much different (leastways, based on our usage). The BG unit rates (then!) were fairly competitive (lower than many other suppliers, though they do have higher standing charges. Possibly I'm doing it wrong, but I didn't see several hundred pound savings (on a total spend of around £1300/year). Peak dual-fuel bills for winter last year were probably £500-600, drops to about £200-250 in summer. BG tend to screw things up in predictable ways, I'm never convinced switching isn't an avenue to problems, no matter what they claim. We spent an age when we moved here trying to convince BG that our meter was metric and not imperial and we really didn't owe them several thousand pounds. I don't want to do that again.
I'm sure plenty of people do pay variable mortgage rates, rates can – and have gone down below what many people are fixed at. You're just self-hedging, the same as the energy companies are with fixed rates.
I am a bit lazy though and not much motivated to spend my evenings comparing quotes to save relatively small amounts of money – though admitted telling L&G to bugger off recently on my house and contents insurance saved me £600+ (mostly because they pissed me off with epic premium hike and when we called them, basically gave us a wevs).