Still haven’t seen any reviews of the Dolphin
If you mean on here, maybe none of the contributors to the forum have stayed there. In general, visitors to London that are regulars on here prefer chain hotels, particularly Premier Inn, in that price range.
The reviews on Google are pretty much what I'd expect for an independent hotel in that budget range; some give it five stars and it worked out fine for them, others have some complaints. It looks like the sort of place where you'd need to be prepared to roll with it if things weren't quite perfect because it's relatively inexpensive. The chains have a degree of consistency maybe that a small independent might not?
I'm not a lover of neighbourhoods dominated by a large railway station. The Dolphin looks to be on a nice quiet square away from the traffic though.
A little further west is nicer, once you head towards Bayswater and Holland Park. On the other side of Hyde Park, the area of South Kensington may have some similarly priced independent hotels, but may be more characterful as a location. You'd just need to pick somewhere that is an acceptable distance from a tube station for you. We talked in another thread about Pimlico as a nicer alternative to Victoria.