I have some experience with logistics of elderly parents, trains, cars, flights etc and creative thinking. Few things spring to mind:
1 - would it make more sense for you to do Xmas at their house so you travel to them and they can stay put in the house they know and are comfortable in. If you were very clear you are bringing all the food/supplies and they are not to do it all, could this be an option eg to bring Xmas to them?
2 - a compromise could be a flight to say Southend (nice small local airport which makes pick up much quicker and easier than somewhere massive like Gatwick or Heathrow) and you drive there to collect them - under 2 hours so not horrific.
3 - are there other family members who can help by doing a leg of the journey eg driving them from Manchester to Nottingham, Birmingham etc and you meet there to collect this halving your journey time. They could stay over at the halfway point if their energies are best with shorter journeys. Have done this with my mum and it worked well.
4 - relatively similar to above but using train if that is an option for them - order them assistance so they are helped on and off train. Look for creative routes where they could break the journey in a smaller place than London for an overnight stay or rest before then doing next leg.
5 - long distance taxi - worth asking around quotes may vary wildly but for 2 people it may not be hugely more expensive than the flights, train or petrol x