Jen's place is excellent. She is an awesome host as well.
Let's start off with the basics. The flat is really well placed. The neighborhood is residential and quiet, yet really close to the city center. Just 5 or 10 minutes walk to Westminster Abbey and Big Ben. Tubes are like 7 blocks away, too.
The place is really clean and well equipped. Nice kitchen, TV, wifi, central heating... Everything you could expect to rest comfortably after a long day in London.
Jen is really responsible and kind. I've always had quick responses from her and helpful ones, of course. She even let me do an early check in without charging extra. Also, she provided guides to get to the place from virtually every place of London. Even local guides and tips, ranging from which theaters to visit, local groceries, plenty of things!
The only thing that I would mention to visitors is that the neighborhood is really quiet, so you don't have pubs or restaurants nearby. If your goal is to have great dinner outside everyday, you would have to walk a few blocks. For me, this was a good thing because I like quiet places, specially after a tiring day. That's the only thing that I would point out.
Other than that, expect a really good place with a very responsible and helping host, at a really good price too!