After returning from MAN-LHR, you will not be able to access the international baggage carousels, only the domestic ones. If you haven't picked up the bags from the carousels after a couple of hours, they are likely to the piled up in a corner, then taken inside the baggage area for reconciliation, a process that usually takes a few days. You can then return to LHR and collect the bags once it has been through the system. They may offer to deliver the bags to you, but if yours were the only bags not collected then they will probably work out what has gone on.