Though I have yet to check all of these restaurants out since being a vegan, I have done lots of online research to compose this list of popular restaurant chains that offer vegan options or have veganisable options on their menu. These are the huge chain restaurants in the UK, however look around in your local area for vegetarian/vegan restaurants and also Chinese or Thai cuisine as these tend to have a lot of vegan options available. This list is as of August 2014, so always check before ordering food and ensure the restaurant knows your dietary requirements, as menus are subject to change. Also, if you're eating somewhere like a pub which doesn't offer any vegan options, phone ahead and ask, most places want your business and like a challenge, so don't be afraid to ask them to make you something!
Beefeater- http://www.beefeater.co.uk/

There's not much in the case of vegan options here, as indicated by the name, however there are some side dishes which could be ordered together to make a meal. The chargilled flat mushrooms, battered onion rings, house salad, ultimate/skinny spicy or plain chips and garlic & herb chimichurri steak sauce are all listed as vegan - so it's kinda like a steak and chips with the mushroom replacing the steak. It's not ideal but at least it's not just a plate of chips!
Bella Italia- http://www.bellaitalia.co.uk/
Currently there are no vegan options on the menu, however some can be created, following this statement from their website; "Vegan options – All our pasta (except the tagliatelle) is vegan, as are our pizza bases therefore you can then choose a pomodoro sauce which is vegan and just add vegetables to your pizza or pasta. These would be plain pizza bases and plain pasta."
Chiquito- http://www.chiquito.co.uk/

Frankie and Benny's- http://www.frankieandbennys.com/
F&B offer a vegan pasta dish - 'Arrabbiata' as well as their house fries, house soup, a few breakfast options and thier mango & lemon sorbets. Their pizza dough and tomato base is also vegan so asking for a pizza with no cheese is also an alternative.
Giraffe- http://www.giraffe.net/

Giraffe seem to have a great number of vegan options on their menu, this includes; olives, endamame, tofu and squash laksa, herby green rice, sauteed seasonal vegetables, skin on fries, chunky wedges, fresh chopped fruit bowl, a sorbet sundae, kids fruit bowl, hummus & grilled flat bread and Juliette's healthy salad. These are not only great options but are relatively healthy too!
Gourmet Burger Kitchen- http://www.gbk.co.uk/

Hungry Horse- http://www.hungryhorse.co.uk/

Mcdonald's- http://www.mcdonalds.co.uk/
Not that this is really a place vegans would generally want to eat at, but minus the mayo their 'Spicy Veggie Deli' - wrap or burger seems to be vegan, there are no animal products listed to be in the burger and it is approved by the vegetarian society as it is cooked in the vegetable chip oil, rather than the same oil as the meat burgers. The chips, ketchup, regular BBQ sauce, side salad, carrot sticks and many of their drinks are also vegan. Stay away from the frappe's and fruit smoothies as these contain milk, however the kid branded 'fruitizz' are vegan.
Nando's- http://www.nandos.co.uk/

Nando's veggie pitta is vegan (ask for no mayo), they also have salads, appetizers, a few sides - including Ratatouille, veggie strips for kids under 10 and a passion fruit Gelado which is listed on their website as vegan. They have a wide range for veggies and vegans which I'm sure you're all pleased to see and in stores they have a full list of the ingredients in every dish to check allergy/dietary requirements before you order! Nando's also have a separate prep and cooking area in their kitchens for veggie food.
Pizza Express- http://www.pizzaexpress.com/our-food/
Pizza express have a few starters which are vegan, as well as their own vegan pizza 'Pianta', which I have had twice now & would highly recommend, dough balls (garlic butter subbed for garlic oil), side salads and a few sorbets and lollies for desserts. They also offer gluten free pizza options and many of the pizzas can be ordered without cheese for a fully vegan option.
Pizza Hut- http://www.pizzahut.co.uk/

Pret A Manger- http://www.pret.com/

Prezzo- http://www.prezzorestaurants.co.uk/

Prezzo have many options on their menu such as their; ciabatta loaf, focaccia bread, tuscan bean soup, spag napoletnana, penne arrabbiata, house fries and tomato & red onion salad which are vegan. I cannot find any information on whether the pizza bases are vegan so this may be something to ask when at the restaurant.
Starbuck's- http://www.starbucks.co.uk/
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Starbucks have vegan coffee options (made with soy milk) as well as a few other food options, including porridge (made with soy) which comes with a choice of brown sugar or a fruit or nut medley for toppings, a 'moreish meze salad', lollipops, aftercoffee mints and popchips -salted or salt & vinegar. Many of their coffee flavourings like caramel drizzle (used to finish off drinks), pumpkin spice, protein powder, java chips, and white mocha are NOT VEGAN, so AVOID these flavourings. However, the mocha syrup IS VEGAN, along with the hazelnut and caramel syrup, so a soy hazelnut, soy green tea or soy mocha frappuccino (without whipped cream) would be suitable, along with a soy latte, vanilla soy latte, peppermint mocha soy latte, a black Americano and a soy cappuccino would also be great alternatives.
Wagamama's- http://www.wagamama.com/our-menu
Wagamama's offers 6 dishes on their menu advertised as vegan on their website which could be ordered with rice, including a raw salad in addition to a range of fresh & in some cases raw juices and a fresh fruit ice lolly. There are also a few other dishes which could be ordered without the egg to be completely vegan. A new warm chili tofu salad has also been added to the menu, I ordered this the other day with a portion of brown rice and it was delicious!

Wetherspoon- http://www.jdwetherspoon.co.uk/

Yo! Sushi- http://www.yosushi.com/

Yo sushi have so many options on their menu which are vegan, including; miso soup, vegetable firecracker rice, vegetable hand roll (no mayo, no tamago) vegetable yakisoba, pumpkin korroke, vegetable gyoza, kaiso salad, cucumber maki, avocado maki (no mayo) inari and chive futomaki, endamame, kaiso gunkan, aubergine salad, anko mochi and fresh fruit.
Zizzi- http://www.zizzi.co.uk/
