Everything was great!! Fiona & Michael contacted me a few days prior to my arrival and suggested various ways of transportation from Heathrow to their place. The place was exactly like the picture. Despite its location semi-underground, their place was very warm, clean, and decorated lovely. We stayed for 4 nights and throughout our stay, both Fiona and Michael were fast with communication (via txt msg or airbnb chat). My family and I were very excited to have had hosts who were very nice and welcoming.
Now, the location wise, the walk from their place to the Underground was about 5-7 mins (Balham station - zone 3). It took about 45-50 mins to get to Central London (stations like Westminster, London Bridge, etc.) during rush hour (7:30 AM - 9 AM; Quick tip: the transportation tickets are more expensive during rush hour so avoid this time!!). Fiona's place is also 3-4 mins walk away from Waitrose (Equivalent of Safeway/Publix for the Americans out there).
Overall, we would not hesitate to choose Fiona & Michael over other hosts if we were to come to London again!