We had a great stay at Karen's place. It is a solid 15-20 min tube/bus ride to the center of London. Elephant & Castle is an ok area and Karen some good food suggestions, but it's certainly not buzzing nightlife.
The apartment was quite small but very welcoming. The bed is not he most comfortable and very low to the ground. We traveled in July and it was quite hot in our room with no fan. However the room was quite large and fully equipped with breakfast/coffee/tea supplies. Bathroom is shared but never had an issue with availability.
Overall good experience!