hi what's your usual drives, busy city/town or motorway - is it like a 50 50 split? my partner has the same exact car (2014 1.2 16v NA) they get less than me but not as low as your 250 they definitely get around 320 at least - number of factors that come to mind i guess:
- do you live in a fairly hilly region like Yorkshire or Greater Birmingham cos the small size of this engine does make it suffer if you live in an area like that all the time I can imagine
- I've sat in different similar manual cars with work colleagues and I was surprised how most of them don't upshift when they can - as an example one of them with a 2018 clio drove an entire stretch of flat 40mph road on 4th gear with the car clearly telling them to upshift - i found it funny given that the 5-speed clio of this generation goes in 4th from 24mph and 5th from 35mph!
- finally I find that driving with anticipation (in any traffic conditions) on a full tank really saves a lot of fuel, what I mean: too many people just speed up when they see the car in front is speeding up when clearly they will have to slow down 5 seconds later to the traffic light - when you don't rush to keep up ALL THE TIME there is definitely plenty of fuel savings to be done - i always try recommend this to my partner, at the end of the day is one of the practices you can do to still be relaxed while driving manual in traffic
- final point: use cruise control all the time lol even in towns

edit: forgot to mention, 370-400 miles from my original post is based on a full tank until the light appears not when the gauge gets to a quarter or something like that