Do you really think that the taskers would schedule 4-5 hour ground time for crew change if there was another way within the given resources? Airlines (proper ones) would have more crews going round a slip pattern so that a fresh one would pick the aircraft as it went each way. I'll bet that what is happening is that the passengers have to wait in the terminal whilst the crew that brought the aircraft from the UK have their minimum rest period before picking up the inbound sector. If there were more crews then an extra couple could be chucked into the slip. To the poor passenger it always seems appalling but (nearly) everyone does their best to make it happen with limited resources. The taskers have worked very hard to try to make the KBL run work.
One could then go on to criticise the minimum rest requirements, but don't forget that these crews are sharing the same airspace as public transport operators. When you buy a ticket for your wife and kids to fly you expect that operators using the same airports and airspace will have roughly similar regulations and observe a duty-of-care towards other carriers and their customers.