hi and welcome. i used to live in Stratford and have been to London a bunch of times. you have more options there than i do here in North Bay as far as eating out goes. there is an awesome vegan restaurant called Veg Out that you absolutely MUST check out. sounds wierd but get the fish and chips, seriously! omg
and the owner is super nice.
i think there's also a veg place on dundas but i don't think i've been there.
oh and if you haven't been to ben thahn you must, it's viet-thai with a really great selection of vegan stuff. i think there's 2 locations in london.
as for what to eat and what to avoid, hit the vegetarian and vegan sections and the food section, there's tons of info and recipes and ideas.
but basically as long as you eat enough calories in a day and a wide variety of natural foods, fruits, veggies, grains, etc, you will have enough of everything. the idea that vegetarians are nutrient deficient and starving to death is such b.s. the only thing you need to watch out for is b12 but if you eat any meat substitutes like veggie burgers, vegetarian chicken substitutes, that sorta thing, and any soy products like soymilk, soy dairy alternatives etc, they are all fortified with b12 and other stuff so you don't really have to worry about it.
just don't fall into the trap of relying on these convenient meat alternatives, they are very processed and full of junk (but still better for you than meat) but now and then they're great to include in recipes you loved before going veg so you don't have to miss out on anything you would normally eat.