Spent a week here with my two teenage boys. Hampstead is a fantastic neighborhood. We felt very safe there. Lots of cafes, restaurants, and stores close by, and the Underground station is a 5 minute walk.
Be aware that the restaurants and pubs in Hampstead are on the expensive side. It's an expensive neighborhood.
The flat is not actually on Oriel Place as the listing shows, but it's not that far from it.
The flat is actually nicer than the photos indicate. It's well cared for and has lots of nice little touches. There is no TV, but we didn't miss it - my kids are prefer to be on their phones, and the flat has wi-fi. The kitchen has the basics - pots, pans, plates, bowls, silverware. We mostly ate out, but had a few meals there. Could have used a vegetable peeler and a colander, as well as pot holders. But overall, quite functional.
I think location is most important, and this is a 5-star location, IMHO. Close to the center of London, and in a very walkable neighborhood. It's small, but comfortable and clean.
Quick tip: if you stay here and are going to the Westminster area, try taking Bus #24 rather than the tube. It's slower, and the bus stop is farther than the underground station, but it's a scenic ride.