What is the average UK household consumption/bill?

You can find Ofgem’s latest consumption figures here. https://www.ofgem.gov.uk/gas/retail-market/monitoring-data-and-statistics/typical-domestic-consumption-values 

It is said the average home in the UK has an average energy bill of £1,326 per year – which is split between electricity and gas. But of course, that varies. Your home, with our energy saving equipment and state of the art technology, should perform much better than any ‘average’. We’ll start by using the average for homes just like yours, and then build up a picture of your actual usage over time.

Here is the Greenage guide to calculating how much energy your home could use. https://www.thegreenage.co.uk/how-much-energy-does-my-home-use/