On surrounding roads there's one (with loft done) that went (recently) for £25k more than she bought hers and another (no loft but comparable in size) that is on for 10k more than she paid. The houses on her road that are £900k plus are bigger than Lily's and the area doesn't have a huge amount going for for her house to be worth that much (post loft) in the next few years. Most people will just cross over the A406 to the area slightly north of her where you can get £900k plus houses which are much bigger, with drives and bigger gardens closer to the park and near a station and a high street. She lives in an area where there are still loads of houses that are in the state hers was in when she bought. So there are still fixer uppers to be had and people tend to go for those because they can put their own stamp on it.
I live in Zone 3 in North London and a lot of people I know who bought in 2017 (which is when Lily bought) haven't made a huge amount on their properties because prices were really high back then - they've fallen a bit since and gone back up during the pandemic. Also, families are leaving London people now that people realise they can work from home and afford much more for their money elsewhere.