Oh she's nearly 8 and a bit of a tom boy really. I should also probably have mentioned that this room is also shared with her younger 5 year old sister, who is more of a princess girl. I had hoped to create some symmetry in the room so ideally I need to find a balance that works for both girls. Wall art stickers could work well. Thanks!

the high quotes are probably due to current timber shortage. most timber seems to be atleast doubled in price at the moment! I think your alternative is looking great though! to answer your question, if the tv inset and the shelf are the only sections not covered by a door I think they should match, so if going grey behind tv i would probably go grey in the shelf bit too

I would do the whole wall no question. If you get a good carpenter there are some fantastic examples out there. Build a low cupboard for storage all the way across and then cubby shelving with some of them behind closed doors and backlighting also looks amazing. Search on Houzz for examples. Also look at the Modern House website. Lots of London property for sale has amazing examples. The sky (and your budget of course) really is the limit. The radiator could be moved or disguised. https://www.houzz.co.uk/hznb/photos/elegant-grey-media-unit-hampton-london-modern-living-room-london-phvw-vp~129551903

I'm not sure that the three items you are planning fit well together. A media wall is usually but not always custom built and can cover the entire wall with storage and shelves and house the tv. You could wallpaper the wall first but it would be largely covered by the units and shelves. Where do you envisage this miirror exactly? I'm not sure what you mean by wall length? If you mean full length then putting a mirror next to the windowseat you are building will reflect the garden nicely. What sort of style are you hoping for?