Westminster and RBKC, the 2 most expensive and wealthiest boroughs in the country, have the cheapest council tax in the country, @Bubblebubblebah.
Lots of people have given you good advice about potential issues like the EPC/service charges/parking/headspace etc @Danni677 but I think the real reason is simple, it's the square footage. It's priced the same as 2 bed flats in the neighbourhood with the same square footage. Although, TBF, it's probably still overpriced because a lot of the square footage in the master bedroom isn't usable.
The EPC and the lack of ability to improve it is probably an issue for any buy to let investors. Although, if it is listed, it may be exempt from the rules.
Plus, it's with Foxtons, who are notorious for overpricing, which explains the high starting price and subsequent reductions.
Two of my friends have lived on Warrington Crescent at different times. It's a great place to live. The gardens are amazing. That flat is tiny though... It would make a fantastic 1 bed flat, and an ok 2 bed flat.
Why do you need 3 bedrooms? Do you have DC or do you need space to WFH? I don't think it has enough space for 2 adults + 2 DC. It could probably work with some reconfiguring and dual use spaces if you just need home office space and a/or guest room.