You might find you need to make a special food shopping trip up to Hendon/Golders Green!
However... have you tried Whole Foods at Kensington High Street? For a short time I worked in the Whole Foods in Giffnock, just outside of Glasgow, and they had TONS of kosher & Manischewitz stuff. I don't know if that's purely because my shop was in a Jewish neighbourhood, or if all Whole Foods Markets stock the stuff... but it could be worth a look.
The Kensington High Street Whole Foods is utopia. I can't say that I've tried looking there because it's been a WHILE since I was there last. I've been back and forth so much - used to live in Wimbledon, now back in the states, about to move back PERMANENTLY (!) but to the Guildford/Woking area in Surrey. I can always use an excuse to visit that neighborhood, though! And yes, Golders Green...
It would be amazing if Whole Foods managed to open a store around where I'll be living - or even better, Brighton, where we're house hunting

Not only am I looking for the random Jewish item but I'm a vegetarian with lots of "granola" wants (kombucha, dr. bronners, raw foods, essential oils, non-soy based meat alternatives - thank god for quorn) and I find that one has to parse things together between Holland and Barrett, Waitrose (if you're lucky) and the odd "free from" item in the regulars - unless you're lucky enough to have a nice health food shop nearby. There is a Food for Thought in Guildford, but it's damned expensive. Whole Foods isn't cheap, but it somehow beats the little stores in price by the sheer enormity of its operation. Not very indie of me, I know, but these are desperate times. I've spent so much money on associated visa and solicitor costs that these next 6-12 months are going to be quite tight...
Anyway, thank you SO MUCH for responding!