trblsm1:
I would suggest for starters you select the closest type of house or "low" usage in the sign-up. What's more important is that you manage your account whenever a statement arrives by submitting a customer reading and if necessary make provision for paying more. I understand you want to have an accurate Direct Debit but that's difficult without accurate consumption. Also it's much easier to increase the monthly payment at any time but it's very difficult to reduce it except at the 12 month review and even then it can be difficult.
Whatever payment is set, you are bound to be ahead until the winter period.
Don't know about British Gas online billing but the Scottish Power online billing system calculates the account balance immediately you submit online meter readings. You do not need to wait to be asked. Every month if necessary so there need not be any nasty surprises.
Tip, always submit your readings just after the Direct Debit is collected.
Looking forward, once you have accurate annual consumption figures the easiest way to get an accurate Direct Debit is to switch supplier using the annual consumption figures. Well worth checking anyway that you are still on a good deal.