If it's the sort of cancer treatment which seriously depletes your immune system, then he'll be living in a kind of HMS Bubble for the duration, so seeing him right at the start (before full effects take hold) makes sense
And as even the Royals rarely overfly central London (since the Vauxhall crash even they go to Battersea normally) it looks as if there was a considerable effort, from the Palace end, to facilitate the visit.
Though as it's half term next week, I was wondering if the little Waleses will be having a break at Amner (their mother might be OK to travel there). They might be able to see the King outdoors, and with the weather forecast as it is, chuck a few snowballs at him!