We also switched from Ebico to British Gas and they only asked for the reading a good while after the switch date and that was by letter (paper through the post). Their site really is, in my opinion, pretty awful but it is possible to submit a meter reading and here is one way:
Log into your account. Go to "Help & support" and then type "Meter reading" in the search bar - you will then get "Submit meter reading" as a choice. You should manage to negotiate the way from there.