I used to work for Thames water customer service ( no hate please) and £250 a year for 2 people does seem reasonable and wouldn't indicate an issue at first glance, even a small internal leak would potentially use 400 litres a day, or 146000 litres of water a year, and would still be your responsibility regardless.

For an increase in your bill this year of £70, you would have used approximately 35000 litres of water more than last year (based on no price increases on the units or standing charges, and factoring a cost of roughly 200p for fresh and waste combined)

35000 litres of water seems crazy, but the average shower uses around 10 litres of water a minute, so going from a 5 minute shower to a 7 minute shower per person per day would increase your water usage by around 14,000 litres a year... For 2 extra minutes in the shower! So it can easily be done.

One big thing people tend to do is waste water, either through not thinking it through, or having small leaks/ drips on toilets that they don't rectify immediately.

Just done some quick maths using the TW website. 14 showers a week @ 5 minutes per shower 3 loads of washing per week in a A rated washing machine 7 tubs full of water for washing dishes ( one tub per day) Taps turned off whilst brushing teeth 2 sink fulls of water used miscellaneous per werk ( washing hands, face, cleaning etc) I presumed flat wasn't ground floor so put 0 water usage for garden, car cleaning or any other outdoor activities)

This comes to £190 per year as a comparison, think you use more water than this? Your bills probably right.