That 2500 is what has been reported as the estimated average annual household gas and electricity bill; bit it would be interesting to know what the original calculations had really said, and how they calculated the average or mean, whichever it is.
I am not a statistician, but I do know figures like this can be horribly skewed if they are merely arithmetical averages or means. (I know average does not equal mean, but I am not even sure of their very important difference).
Even now £2500 a year seems high for most households, but like many costs, is it influenced by the much larger values from the much smaller proportions of those to the overall count of values.