yes, definitely bring as much travel food as you can! i spent my semester abroad in london and there is practically NO vegetarian convenience food. just lots and lots of pre-packaged sandwiches made of white bread, slimy meat and mayo....yuck. perhaps you could pack a few luna bars or other easily transportable snacks.

on a sidenote, if you're going to be in london, definitely visit Food for Thought in covent garden. sooooo good!

Well I was only there for one semester so I probably didn't find all the good spots. Plus I was in the East End (Queen Mary) so maybe there is less selection there. I can't wait to go back though and see what I missed!

i'm from the UK and have travelled to north america a few times in the past year- its a 6 hour minimum flight, right? from experience, i've found it best to pack a lots of  little snack items, like mixed veggies, diddy cartons of rice milk and shakes, etc, ricecakes with cashewnut butter, little pots of mixed rice salad, mixed bean salad, soup, etc- pretty much anything that you can fit in a pot with a lid is good. a lot of airlines will provide a pretty nice vegan meal if you request it in advance- look at their website- some even offer a few vegan options (air canada, for example, offers asian vegan, indian vegan, and western vegan options!)
i'd be very careful about what you plan to take off the plane at the other end, british customs will remove any plants, and fruit and veggies from you, and tell you off for carrying it, at the least.
i've packed favourite vegan food products that i can't get at home (the uk) in my suitcase, things like sealed packets of vegan cookies, etc, and bought it with me, but if customs asks you what you have with you food wise, be honest or you'll get in trouble if they find out otherwise, they've let me through with prepacked vegan foods before (i said i have allergies and needed to make sure i had something safe for me to eat for my first few days in a different country, and they were ok with it).
british vegan food is pretty good if you are in the right area, especially places like brighton, london, and glastonbury- where are you going? most towns have a health food store, (holland and barrett is a common chain, or a store like julian graves which sells bulk fruit and nuts, etc)and they'll usually offer tips on good places to eat out locally.
mcdonalds fries and hashbrowns and salads are vegan in the uk if you want/are stuck with junk food, unlike in the states where they contain cow, and you can get vegan options in way more fast food places in the uk than the states, i've found. mcdonalds has vegan jello (jelly) in the uk, and places like burger king, and even kebab shops in the uk, offer veggie burgers!
also, english supermarkets have come a long way in the past few years, most have a lot of vegetarian ready meals, some of which are vegan (sainsburys has a lush vegetable dhansak in its indian section!), and a lot of supermarket chains (including Sainsburys, and Tescos from the top of my head!) have a dedicated 'freefrom' section which contains allergy safe foods like rice pasta, along with organic lines like soups, and healthier snack options like granola bars, packets of nuts etc. you'll find tofu sold under the 'cauldron' brand in the supermarket (its weird but still good) and a range of veggie and nut burgers in most supermarkets frozen sections too, as well as tonnes of the famous british quorn (check for egg protein in this though).
also, be aware that stevia is illegal in the uk, if you use this, you can't bring it into europe- well, you can't if you get caught, * wink wink* lol.
have a good trip!