the 2 for 1 deals. I didn't use it myself since I went alone, but from what I understand you need to print out the coupons for the places you want to go from the website. once you're in London, when you come to buy the tickets you show them the print outs and the paper travelcard and you should get the discount.
if you visit tripadvisor London forum, it was explained there many many times. a quick search should get you the answer.
Stonehenge is a reasonable day trip from London. You could either get a train to Salisbury then a local bus or trip, or take a pre-arranged tour from London which would be more convenient but probably more expensive. Personally, I think Stonehenge is one of the most overrated tourist destinations in the entire world but it's certainly possible if you want to do it.
To get from Luton to Montague Street you need to take the Thameslink train from Luton Airport to Kings Cross St Pancras, then the Piccadilly line to Russell Square. It's a 5 minute walk from Russell Square station.
For cheap eats, Chinatown is a good bet and quite close to where you are staying, particularly the all-you-can-eat for £6 deals or whatever. Indian Veg in chapel market by Angel tube station is also very good value and tasty, all you can eat for under £5.
Thank you for the answer. What about shopping? I've heard that it's great place, where u can buy clothes for supercheap prices.
Christina, have a look at -snip- - it's tailored for shopping on a budget.
I guess you're back already - how was the trip?
[ Edit: Sorry, no promos please. ]