Air India economy class LHR - DEL - NRT £ 393, several dates available in September. Not the best of airlines, but a cheap price to fly to Tokyo and you get to fly the 787 Dreamliner.
I haven't checked any of the routing rules through ITA. Visa is probably an issue. I'm in the process of requesting one for India...
cheers - let me know how painful the visa process is (admittedly it'll be slightly different for UK probably)
I'm doing it through a company called E-visum, just filled in all the required online forms. Also had to make a scan of my passport and a passport picture, uploaded those and they'll arrange everything for me. Did the same with them for my upcoming trip to Sri Lanka. Hassle free so far!
Getting an Indian visa is very simple since they switched to the eVisa system. Why are you paying some company to do it for you?!