Technically you can just check them in for LHR-NRT, but if you try and do this before the LHR-AMR-LHR segments it seems to require agents to make many phone calls, ponder the meaning of life etc. Folks here however have been successful with it.

Worst case is leaving your bags in left luggage at LHR and checking them in after your LHR-AMS-LHR segements. I've never done this but I believe there is some sort of storage facility.