I am taking two 13 year olds to London on Saturday, DD plus her french exchange. I am going to do a few tourist sites, probably Trafalgar Square, Buck Palace, then across the Thames and the South bank. I would like to take them somewhere a bit different for lunch, instead of just Pret or similar, not necessarily with typical English food but something memorable. Cheap would be good. I think we'll be walking mostly but a short bus trip would also be fine.
And if you have any other must see ideas, that would be great, ideally cheap/free! Harrods food hall is impressive, but a bit of a trek.