London Pass

You can get the two for one passes at the Visitors Center as well as A LOT of information. You can also buy your Heritage Pass if you are going to be in England and Wales for a period of time. It covers the National Trust sights also. Blenheim Palace is on the Heritage Pass if you plan to go there. It was a great experience.
The two for one pass covers the Tower of London.
Just don't forget you have the 2 for 1 and use them!
I'd go to Bath for a day rather than Stonehenge if that's all you are going to do. You would have so much to see in Bath.
I think you can get a lot of the coupons off the internet. Do a Google search and download them BEFORE you go.
Also some of the coupons are in the booklets you can get from the Visitors Center IF you don't print them out before you leave on your trip. We LOVED England and Wales.

Linda - Karen posted a thread about 2for1 for the British Heritage Pass.

Go to www.visitbritain.us (has to be the .us site, I couldn't see it if you went to .com) and look on the right, about half way down for the great deals section. You'll see where it says "buy one heritage pass get another free". Once you click on it it takes you to a page that has a lot of deals. It's about half way down, about number 10 or so where you will see the heritage pass. Just click on it and it opens a new window with the ordering information. It's only the 4 day pass thats bogo. They did tell me on Feb 27th that the offer was still available and when I ordered 1 they would automatically send me another. I just placed the order today and total wise it was about $64 with shipping. Let me know if you need anything else.

The London Pass is quite poor value for money, I think you'll be hard put to make any worthwhile savings at all.
I think you are getting confused with the 2 for 1 offers done by the train companies for HOHO buses, Tower of London etc. The vouchers are at this web site (http://www.daysoutguide.co.uk). To get these very worthwhile offers you need to buy a ticket from a train ticket office (not London Underground). For visitors this means buying a Travelcard pass for their Underground/Bus public transport around London and not buying the Oyster Card rival product. The important factor is that only Travelcards purchased from a train ticket office at the mainline stations in London are valid, not all the other ticketing outlets.

Are you going to arrive and depart from Heathrow? If so, do you plan to take the Tube to London? If your answer to both questions is "yes" and since you're going to spend the major part of at least one day away from London, then it may be to your advantage to buy 1-day, zone 1-2 National Rail travelcards for any day(s) you plan to visit places covered by the 2for1 offers. Use a pay-as-you-go Oyster card for getting from Heathrow to London, other days in London, and for getting back to Heathrow. The Oyster computer will automatically give you the best fare and on any given day will cap at no more than 50 pence less than the cost of a 1-day travelcard for the zones in which you travel that day, no matter how many rides you take. When you return to Heathrow you can get a refund on your deposit and on any unspent money you have loaded on the card.

If you don't want to do all that juggling, then buy 7-day, zone 1-2 travelcards from a National Rail station for all your rides around London. They're not sold at Heathrow so you'll have to wait until you get into London to buy them. Single tickets on the Tube from Heathrow to London cost €4.

Linda,

We're a group of four currently in London and we took full advantage of the 2 For 1 offers through www.Daysoutguide.co.uk and the National Rail service and have saved over 100.00 so far.

We had numerous errands to run our first day in London so we purchased a one day travel card (with the national rail emblem) from the station at Heathrow Terminal 3.

We then purchased a 7 day travel card from St. Pancras (but any national rail station will offer them) and we used our two for one passes for the following;
Tower of London
Westminster Abbey
Churchill Museum and Cabinet War Rooms (Excellent!)
British Museum's Babylon Exhibit
Kensington Palace (which I would normally avoid but we had friends with us that had never been)
St. Paul's Cathedral (which included the Whispering Gallery
The Household Calvary Museum

We printed our coupons before we came but I did find brochures that provided the preprinted 2 for 1 offers though they weren't available at all National Rail stations. Bath and Bristol also have 2 for 1 offers.

Have a great trip!