Thank you all - very helpful. What I've got is that the M25 will be horrible (so no different from the other 364 days - but fortunately a road we don't need to go near), shopping centres are going to be busy, and Reggie is going to do a short 50-mile pootle across some of Southern England's best countryside with a hangover.
Actually the bit about shopping centres being busy is particularly helpful. We avoid them at the best of times, and have never understand why people pile into shops on Boxing Day to save a few quid on something they don't really want. Our route will be M40 - M42 - M6 toll - M6 - A74(M) - M74, a monument to the tedium of long-distance motorways. Looking at a list of large
shopping centres and a
map of outlet stores we should be OK once we've got past Bicester Village - unless anyone can spot any other potential gnarls. That might give us an extra hour in bed. I'm thinking quick coffee before we leave, then breakfast as it gets light just north of Birmingham, coffee at Tebay and late lunch when we arrive.
This really isn't my idea of a great Boxing Day, and if there was any other practical way of doing the trip we'd find it. But we'll have an 80-something with chronic arthritis with us - and it's largely for her that we're thinking of making the trip. As long as we can break regularly for fresh air stops she'll probably sleep in the back of the car. She wouldn't be able to cope with either the train or the plane, even if there were reliable trains on Boxing Day.