Wotcha and welcome - The 1.5 suffers from a power to weight ratio challenge - small engine but by comparison a large heavy car to lug around
this parkers link gives 37mpg and tank range of 488miles
https://www.parkers.co.uk/subaru/impreza/hatchback-2007/15-r-5d/specs/
This I would say would be possible with a light foot on a long motorway run but urban driving I would say a lot less
My 2.0 RX NA returns 28mpg ave with urban driving 37- 42 on a long run
Do you have the dash ave mpg indicator. Has it been serviced regularly - new filter, plugs etc. If it has been run on rubbish fuel the injectors may be partially blocked.
Try adding a bottle of injector cleaner for the next few times you fill up - If you can get hold of BG44K that is good stuff but tends to be from a local garage not in shops, or Millars is another one I am using when I cannot get the BG stuff
http://www.millersoils.co.uk/products/0/580