The reason for this offering is, that there is not enough space to return cater hot meals ex LHR on the DME A321 flight.
When the flight went onto the 747/767 mix the flights were return catered and the contract with DME catering was terminated.
Now the A321 does the night flight and some daylight ones no new contract has been made, thets why the (poor) offering of the return catering of cold sandwiches in WT happens

Most Eurofleet Band 4s get catered at the outstation and so do the Midhaul flights, hence they can offer a hot meal in Traveller on those flights.