You've been given a lot of shoddy advice here sadly. While many posters may not have heard for them, it's not totally unheard of for an area of the house to have a separate meter you need to top up yourself. It's most likely to come up if you live in an annex and your LL wants to know exactly what electricity you're using so you pay your fair share.
Thing is, this meter is likely NOT administered by the electricity company, your LL will probably be named on the main bill that will cover all electricity usage for the house and will recover the money for this from your sub meters.
The important things to check with this sort of meter arrangement are:
That it is only linked up to electricity used in your room. Houses can be wired up in all sorts of ways, you need to turn everything off and check the meter doesn't still increase, if it does, you're paying for more than your own usage.
Secondly, the LL is responsible for setting the rate on the meter, however, they're only entitled to set the rate at the same rate they are charged by the elec company. This can be harder to work out because likely the meter will only charge at one rate while the electricity bill may have a standing charge and different units charged at different rates, the rate charged by your submeter is required to match (as closely as possible) the rate charged to the LL. You can ask them for a bill or something with their tariff on to verify this if your bills end up being huge.
As to how much your bills will be, it completely depends on what you're using! Are you sitting in all day with your huge plasma tv on? If you're sensible about what you're using, considering that it isn't going to include lights, cooker, electric shower, washing machine etc. I can't see your bill being worse than about £5 a week depending on what you do actually do in your room. My bill for my whole flat is under £10 a week in the summer and that includes all the things I mentioned.