Most seem to be run by dubious characters with no hygiene.
At least those that sell those disgusting nuts...
We don't really have many of the carts I see in American movies, and the few street vendors you encounter have the (possibly undeserved) reputation of being poorly regulated and liable to sell low-grade, or even toxic condemned processed meat products. Search TA London Forums (box above) for the very many budget curry restaurants or pubs
There are very popular mobile food outlets in Leather Lane most days. I have never eaten from them as the many fixed shops have offered more than enough choice.
"Food carts" to a Londoner mean hand pushed trolleys almost entirely run by gangs and never sanitary. Never, ever buy from one of these. The food is never thrown away at the end of the day and is just reheated. Also the seller never bothers to take a break away from the cart ! I will leave the consequences of that to your imagination.
The 'food carts' - such as they are in London -are to be avoided and have been largely hounded out of town for selling dire, unhygienic food (loose description).
There are however some food stalls which sell good cheap, hot food at lunchtimes in Exmouth market, and Lower Marsh and probably other similar locations in London. The latter has an excellent stall which sells Thai curries at lunch times. There are sandwich shops all over the place, supermarkets (some of which sell hot takeaway) and chains like Leon, Pret and Eat which do sandwiches, salads and soups to eat in or take away.
And of course there's Borough Market on Thurs/Fri/Sats.
Very glad I asked! Do office workers tend to use the food stalls or just bring bagged lunches?
Goulston Street just east of the City has a great collection of vans and stalls selling hot food on week-day lunch-times.Edited: 2:13 pm, August 12, 2011
Can't say I've seen a single food cart in 7 years here, save for one hot dog cart whose owner was legging it from approaching police in Holborn late one night. I must go to the wrong areas of town.
What do we do for lunch? I usually go to small cafes for a panini, maybe to a sandwich shop, sometimes a noodle bar or salad bar like "Tossed" and sometimes the pub. If I'm not so flush I'll take a packed lunch.
I go to the caf. The thai stall on Lower Marsh that parkandwalk mentions above is absolutely delicious.
I often find myself arranging meetings around Waterloo as an excuse to eat in Lower Marsh.