This is one of the best AirBnbs I've stayed at! We stayed here for 2 weeks on a work trip. Some details for why I loved it and what you can expect
1) This is located in the heart of Mayfair - one of the best neighborhoods/swankiest n'hoods around. Close to pubs, coffee shops, grocery stores, restaurants etc. Personally loved the walkability to Bombay Bustle, Hakkassan, local pubs, Oxford St etc. Close to various central Tube stops.
2) Has everything you need for a longer stay - kitchen (fridge, microwave, oven, stove, dishwasher, some spices). This was great for making my morning tea and re-heating leftovers or deliveries as needed OR heating any simple items from the grocery store.
3) Does have a washer/dryer (one of the all in ones) so that you can do light loads of laundry during stay - including detergent.
4) I don't know my British bed sizes but "double" is prob closer to a queen/queen+ as I have a US king at home and it felt closer to that. Mattress is firm which we loved b/c this is what we have at home. Bedroom is a great size too. The instant hot water heater makes for taking great hot showers.
5) Yes there is no elevator when bringing bags - deal with it and plan in advance. :-)
Rohin is an amazing host, I would highly recommend this place and definitely look forward to staying here again on my next visit to London.