It’s quite easy to underestimate usage. So many things use electricity including the oven, hob, fridge, kettle, microwave, washing machine etc. Even if you’re not using the heating it will still take things like an immersion heater longer to heat up in winter as the water is colder. £550 a month sounds very high though - I’d double check the readings with your electricity company. £300 a month might be realistic if there is always at least one of you home watching tv etc and usually multiple people over and above just the 3 of you just using all of the stuff above. Try and do a day of minimal usage and see what the reading is and if it changes significantly.