Vegan options in chain restaurants
Chain restaurants often have menus and special diets information (sometimes specifying vegan dishes) on their websites, so check the website if you're planning to go to one. If that doesn't help, give the restaurant a call or an email if you want to check that there'll be a vegan option at the place you want to go to. I find that customer services often don't seem to have any information about what goes into their restaurants' food, but the restaurants themselves should be able to help.
This is not a complete list, but here are some that I know about:
Las Iguanas: Click here for a list of vegan and veganisable dishes
Nando's: The Beanburger and pitta and Veggieburger and pitta are suitable for vegans if you ask for them without the mayonnaise. (source: email from company May 08)
Pizza Express: The pizza dough and the tomato sauce are suitable for vegans and they make the pizzas up from fresh so will leave non-vegan items off pizzas on request. Their pesto is not suitable for vegans. (source: email from company March 08)
Pizza Hut: The Pan Pizza and The Italian pizza bases and tomato sauce are suitable for vegans and you can order pizzas without the cheese (and add extra vegetable toppings as normal). The online delivery order form lets you select "no cheese" as a topping choice. Some of the starters, dips and salad bar items are also vegan. Vegan list here.
Wagamama: The website has a 'dishes which can be made vegan' list and if you ask the waiters at the restaurant they will bring you a copy of the actual vegan list. When I went to the Sheffield one they also had a couple of vegan desserts (sorbet and fruit ice lollies).
Wetherspoons: They have a few vegan options on the menu (mainly one curry and assorted extras, jacket potato with beans and a couple of soups), although they do fewer vegan dishes than they used to and make some vegetarian meals which could so easily be vegan but aren't... Nutritional info pages here.