I lived in London for a year, ( in mid 90's ) and have been back at least once a year since for a visit.
For value, my all time favorite hotel worldwide, is the Holiday Inn Kensington forum. ( about 100 yards from Gloucester road tube stop )

IF you want the to live the "british experience" ( room with saggy bed, small toilet with string which you pull to flush and greasy brekkie) then stay elsewhere.
This place is a large high rise, which is very modern, and very similar to an American hotel. I noticed that most of the US airline crews stayed there.
To me, it is perfect.
Large room. Large modern bathroom. Very comfortable, and very very reasoably pricd.
There is a starbucks about 1 minute walk. Loads of restaurants. ( one of the better Indian restaurants in London is a block away )
Loads of pubs, and small places to eat.

Also, gloucester road tube station is on of the better stops since you can connect to multiple lines ( including Piccadilly line which takes you to heathrow in about 40 minutes ) so your group of 4 can negotiate your way from airport to hotel on the cheap and easy.

book directly on line at the sixcontinenthotels.com website. ( make sure it is kensington FORUM since there is another holiday inn Kensington which is not as nice )

enjoy!