British Gas complaints procedure is very simple and helpful. First: visit their website and the relevant page: https://www.britishgas.co.uk/complaints.html
So first, for gas and electricity, you should call this complaints number: 0333 202 9532 or write to this British Gas complaint address: Complaints Management Team PO Box 226, Rotherham, S98 1PB.
You can also send a complaint through email: [email protected]
If you need outside help with your complaint through the Energy Ombudsman, just Google Citizen Advice: https://www.citizensadvice.org.uk/energy/
You can also use our British Gas complaints form and we will help you in sending it to the right place for free!