I'd be surprised if you can't get good Osechi in London -- there was a Japanese restaurant in the DC area (Bethesda) that took orders every New Year and my family picked up an Osechi set from there several years in a row (along with a platter of sushi to eat while watching Kohaku). Basically same quality as Japan since everything is preserved and flown in. There has to be at least one Japanese restaurant in London doing the same, if not several.