8000+ kwh bill for a small flat for a month???

Gerry1

Make sure your next place has gas central heating.  Electric heating is always expensive, and full price (i.e. non-storage, not Economy 7) heating and hot water is the most expensive of all.

7Phil

Based on the recent consumption: Today - 20476 & 30 Jan - 18816, that's 1660 kwh for 27 days. Based on my energy plan that is a cost of £215 for almost one month.
You've already worked out that you've averaged 940kwh for the 14 months between Dec'18 and now. Based on my plan that's ~£125 monthly average. For "a small flat", it's high! 
We don't know the specifics of your flat - how well insulated and efficient it is. Whether the other appliances are efficient and working well. Are you reliant on a clothes drier? If your fridge or oven seals are worn our then that could be other sources of energy loss.

I think you understand, and are accepting, that you've paid far less each month than you should have resulting in a sudden bill-shock (lesson learnt, true) however it's still worth putting in a complaint. You just never know, the meter *might* be faulty. Sadly you are moving out so I am not sure how it can be checked or proven if you don't have access to the property to let an engineer investigate but you've got nothing to lose. You might even get offered some credit off the total to help out. Smaller energy suppliers are probably more likely to hold their ground though...
Worst case scenario is you buy some extra time while your complaint is processed to find the money to pay off last year's energy use (though you'd have to check whether bulb will give you extra time to pay the bill if you have a complaint open).
Good luck.