http://www.walks.com/ This is the "original" London Walks site - they've been around since the 1960's and we did their Jewish London - Eastend walk. Very interesting and informative. They've got a "Beatles Walk" , Harry Potter, Sherlock Holmes, etc.
Other option -
http://www.secretlondonwalks.co.uk/Bus tours are a good start. And you can always do your own. Get a bus pass, ride from one side of the city to the other. If you see something interesting, get off.
London has gotten hideously expensive - Do pub lunches and ethnic food. The top end restos are going to cost an arm and a leg.
Four recommends.
If they want a really interesting meal (but pricey) go to Fergus Henderson's restaurant St. John. Tony Bourdain has always said he wanted his last meal cooked by this guy. Of course, if either your brother or his wife have "food issues" this is not the restaurant for them, so I've included the web address
http://www.stjohnrestaurant.co.uk/The Seashell fish and chips restaurant near Marleybone Station.
http://www.seashellrestaurant.co.uk/ One of the BEST fish & chips restaurants in London.
And The Surprise - it's a pub in Chelsea frequented by advertising types and Chelsea Pensioners from the Chelsea Hospital - Army Retirement home down the road. From Sloane Square - King's Road (0.3km)
Turn left: Cheltenham Terrace (0.2km)
Turn right: Saint Leonard'S Terrace (0.3km)
Turn left: Ralston Street (<0.1km)
Turn right: Christchurch Street
There's a darts board, a skittles table and you can play cards. Good lunches and good beer.