K.W.
Welcome fellow Brit! I haven't had much joy finding Robinsons or indeed any squash but did find some Ribena at Treasure Island. You can also buy baked beans, heinz spaghetti, chocolate hobnobs, salad cream etc there (albeit at hugely inflated prices!) if you feel you need a fix. World Market also does a small range of similar Brit food including Cadburys. Wholefoods sells Marmite. I haven't managed to find PG tips but i haven't looked as hard for it so it might be out there.
As for formula, can't really help as i haven't used it but i would say to check the label on any you buy as i have heard (and i might be completely wrong on this) that some brands have high fructose corn syrup in - this is not something i have ever seen in the UK but it is in so many things here as a sugar substitute and not something i would want to give myself letalone my baby!
Good luck.