Big thanks to all who have replied so far.
RAC Route Finder is suggesting the drive to be just under 1300 miles, with a driving time of 19 hours, from Calais going the west side route.
With the western route suggested as being the quicker option, along with the fact that I've previously part driven the eastern route, I plan on travelling the westerly path, mainly to see some of France and Spain that I haven't seen previously.
We aim to be in Calais and ready to set off from there sometime between 9am & noon Wednesday.
If I set off at the later time of noon, with a target time of arriving midnight Thursday, that gives us 36 hours to travel 19 hours, sleep, and make a number of stops throughout the journey. Should things not go as expected, or we have good reason to stop and take a bit longer, we have a contingency period that allows us to take extra time, but ideally we would arrive no later than Friday 5pm.
Giving consideration to the above, I would have thought a Thursday night arrival would have been capable, quite comfortably, or is there something/s I'm not considering?
Jaws 101 - As for my dog - his comfort is priority - he is actually the reason we are not getting the ferry to Spain! As the pet cabins had sold out and I wouldn't put him in the kennels. Actually, he is also the main reason I bought my motorhome!

If we need to take longer for his sake, as suggested, we will be happy to do so. Any advice regarding what's best for my dog, places to stop, recommended walks / parks etc are particularly welcome
Baldilocks - My status showing 'expat in Spain' is part accurate, part wishful thinking! Due to other commitments, I will be based there for approx half of the year in total, most likely following a pattern of 1 month here, 1 month away, give or take a few weeks either way. During some of my time away, I will be using the motorhome to visit a number of surrounding countries, I will also use it on occasion when going back to the UK, where I envisage I will spend approx 2 months of the year in total. Any tips for managing such a semi nomadic lifestyle are also welcome!