I think it is a well presented house. The front is tidy and has kerb appeal. The inside is tidy but a bit too impersonal. The back garden is a basic estate house garden so will compare with many others - its a bit empty but I think that is probably the area where spending money wouldn't be worth it.
I would suggest a few tweaks to the inside and then get some new photos taken.
Sitting room - needs a little bit of warming up/personality. I'd get some new cushion covers and a couple of throws to fold over an arm of each sofa in a warm shade - perhaps red kilim or in bright teals. The rug is too small and too neutral - get something bigger that tones with the new cushion covers - places like Dunelm have cheap rugs in their clearance/returns centre. Move the desk out of the window and put in an armchair instead. If it will fit you could move the pink chaise down and accessorise in pinks? A couple of much bigger houseplants would help too. You could move in the bigger one from the kitchen.
Hall is great and uncluttered.
Kitchen again needs warming up - a buyer needs to see it as the heart of the home and a place for happy family meals, so you need something on the dining table - eg a large coloured bowl of fruit or a large vase of flowers. A few herbs in small pots (Flying Tiger does nice metal ones in bright colours as do Primark) on the windowsill. Maybe the tea and coffee canisters on a bright tray in the corner. You need to tie the colours in so eg a blue bowl of oranges on the table, herb pots in various shades of blue etc.
Bathrooms - put all the plastic bottles etc away in the undersink cupboards. Buy a couple of nice new fluffy towels that only get put out for viewings.
Bedrooms - you need to make the two with sleeper beds look very different - a brightly coloured IKEA desk chair the younger one and some much brighter bedding - get rid of the drinks bottles (its not a bar) and perhaps replace with some pots of pens and colouring pencils etc. The other one should be styled more as a teenage/gaming room - if there's space perhaps the desk from downstairs so it looks as if there is a defined homework corner and a defined gaming corner. The spare room - I think fine to have a photo with the sofa bed open and one with the sofa bed closed - make it look more welcoming again a bit of colour - perhaps the rug from the sitting room and a throw that can either go accross the bed or on a sofa arm plus a couple of cushions. Get rid of the gold mirror it doesn't work with the house and looks silly on the floor.
Garden - I wouldn't spend money on plants etc but you want a clearly defined area where the family can sit and eat outside - see if someone can lend you a garden table and chairs to stage at least for the photos if not for viewings and put a large low round terracotta pot planted with herbs or violas on the table as a centrepiece - can always bring it indoors to brighten up the kitchen afterwards.
Get the estate agent to label the photos - so living room/bedroom 3/bedroom 4/bathroom 1/bathroom 2 and get the dimensions put on the floorplan.
Not an expensive fix - a bit of money spent in IKEA or wherever will brighten things up without making any big changes.