I've been going to Beatroot for more than 10 years. In this time the format's remained the same....I'm not even convinced they've changed their menu. I like it though - it works for me: wide choice of things to eat, all piled together in the same box. By the time you're half way through they're all mashed in with one another, but that's part of the fun.
Value's good, currently about £6.50 for a large box (I didn't bother looking at price of smaller options!). They don't mark the vegan items but if you ask they'll point out those that are unsuitable. There's limited seating, but several parks near by.I probably wouldn't go there too regularly if I spent more time in London, as I expect it'd get a bit boring after a while. It's good for occassional lunch if you're in a hurry though (or shitfaced on rum, wearing a santa suit - don't ask).
Top tip: If you want more than a couple of sausage rolls; ask for some first, then again at the end. They seem to scrimp on them otherwise. I always go with some shepherds pie and a bit of pasta, then try other things each time.
I've had cake from there in the past, but it wasn't amazing. I'd recommend visiting Mrs Marengos for dessert after.
Oh man, that box looks lush! :D