We can’t switch energy providers that easily; it is a process that takes 6 weeks, although there is a mandate to bring it down 24 hours. But with regards to the previous inhabitants: if you have built up a substantial debt with the energy company, you most likely can not switch until the debt is cleared. Only when you move are you able to say that you are a new inhabitant and thus you need a new provider and account on the meter with whatever energy company you want (and the debt from the previous account owner would not be disclosed to you). It could be something like a debt or miscommunication that would keep them locked forever.
Even if you have a debt, you should be able to go on the cheapest tariff, but dealing with debt that you can never repay, only service, is unbelievably stressful, and many people don’t want to think about it or look.
It is also very likely that the previous inhabitants had a cheap fixed-length tariff, and then simply did not look or did not notice the tariff after the tariff length got switched to the default, and thus most expensive one. Energy companies count on that.
As far as meta-buyers go, that’s exactly what Martin’s Money Saving thing does: you can join their energy club and every year you will be warned when your tariff is expiring, and how they are negotiating club-only deals for members with the Big 5. Comparison sites on the web do that too.
It will take for Smart Meters to be ubiquitous before fast switching can be guaranteed.