I think you need to give it time. There are lots of variables that affect fuel consumption, one of which is temperature.
^ This. And the circumstances cited all contribute to low MPG;
- Low ambient temperature
- Short journey, during most, if not all, of which the car’s engine oil won’t have reached it’s optimum operating temperature
- Stop start London traffic
- increased use of ancillary equipment; a/c, heating, lights. rear window hearing etc.
- New engine that hasn’t loosened up yet.
OP; I find it’s more accurate to calculate the fuel consumption manually; filling the tank each time I re-fuel and dividing the mileage covered by the gallons used, rather than rely on the fuel computer. You should find the consumption improves with mileage as the engine begins to loosen up, especially if you include some longer journeys as well.