There are a couple of different ways that you can contact British Gas.
They have an extensive customer support section on their website with articles covering a range of common topics. This way you could be able to resolve your query without having to contact them. You can find this here https://www.britishgas.co.uk/help-and-support/contact-us
There is also a live chat available on that page
If you would like to call them, the British Gas contact number is 0330 100 0056