The correct way to do it is either to travel Hand Baggage Only, at least for the 2 companions. Or to split the reservation, and for the 2 companions on the new reservation move the LHR-MAN dates after paying the appropriate fee. Or if cheaper, get a refund and rebook.
Otherwise you cannot assume the bags will be checked to LHR only. The agent may do this, but they are not supposed to. If they agent sticks to the rules then those bags will be labelled to MAN and retrieving them in LHR will at least result in delays, possibly a trip to lost property and / or may end up in Manchester on an uncertain date anyway.