How and when you pay can also make a difference to your bill. If you pay by direct debit you will often get a cheaper rate than if you pay on a three monthly billing cycle.
One other thing that you might not realise, if you are (both) from the US, is that British heating systems are designed so that they can be set to work for two periods during the day.
If you are out at work all day you might, for example, have the heating set to work from 5am to 8.30am and from 4pm to 10pm, so that the house is warm when you wake up and warm when you get home from work, but otherwise the heating isn't working.
There is also typically an "override" button that allows you to turn the heating on/off without changing the programing. And most modern systems have seven day programming so you can allow for a different timing pattern at weekends.
Also, IMHO, Americans seem to have their heating set 5º or more warmer than many people do in the UK.