Help with last minute itinerary change due to injury

Windsor isn't particularly convenient for anywhere other than London or Heathrow airport .

York is a very walkable town and very easy to reach by train from London takes 2 hours give or take a few minutes. Assuming you are arriving at Heathrow could you have Windsor as first stop followed by London followed by York?

OP - I don't have an opinion on adjusting your trip, but i'm sorry to hear about your husband’s bicycle accident. Please pass along warmest wishes for a steady, comfortable mending and a smooth recovery to his shoulder.

The original plan around Windsor was to see other places the day we drive there, get there in the evening and check in, and then have one full day in Windsor. Our flight leaves early the next morning so would've been an easy drive to drop the car back at Heathrow. With that not being needed now, I'm not sure how we could change it and still see Windsor? If we wanted to travel to Windsor the day before we leave, isn't that a pretty long travel situation to be able to still have the whole day there after arrival?

You could still do the Cotswolds without a car. The train does go to Moreton on Marsh. It might be easier to stay there than Stow. We stayed in Chipping Campden and were luck enough to find a taxi after arriving at MIM. There was supposed to be a bus, but we found that the times of the buses was a little tricky to confirm.

But York sounds like fun, and less hassle. Its on my bucket list too, so I can't comment on if you'd like it better

A good base for York, which would be more countryside/small town would be Malton- on the hourly York to Scarborough rail line.

There are also buses to Castle Howard from both York and Malton- https://passenger-line-assets.s3.eu-west-1.amazonaws.com/eyms/EY/81-timetable-20260224-8f3a9298.pdf
and Malton is on the bus route on the York to Whitby (via Pickering, North Yorkshire Moors Railway) Coastliner bus route.

Apart from regular buses from Moreton on Marsh there is also the weekend/bank holiday heritage bus route to Chipping Campden, Stow on the Wold and Bourton on the Water- https://www.cotswoldvintagebushire.com/service11/

Windsor to Heathrow also has the #703 bus to Terminal 5 at Heathrow- a basically 24 a hour day operation- https://passenger-line-assets.s3.eu-west-1.amazonaws.com/courtney/CTNY/703-timetable-20250831-cf0d48f1.pdf

York to Oxford (for Moreton on the Marsh) or Reading (for Windsor) you would use the direct York to Oxford/Reading trains- basically every 2 hours - takes 3 hours York to Oxford, 3.5 hours York to Reading. So easier than going via/across London.

Change at Reading to the Elizabeth Line/GWR to Slough, then change to the Windsor branch shuttle train (or the bus Oxford Airline bus Oxford to Heathrow then the 703 bus to Windsor)

I was going to suggest Malton, but I can see Stuart already beat me to it! I definitely agree with that, we had a very nice weekend there without a car last year. It's easy to get to Castle Howard (the bus times didn't work for us so we wound up with a short taxi ride) and you can get a train to the coast- you can get to the Bempton Cliffs to see seabirds (depending on what time of year you go) or head up to Scarborough and towards Whitby. It's a really nice area to visit especially in summer. I hope your husband recovers well!

curious - which shoulder? If the left one, could you consider splurging for an automatic? Can mom or son drive?

How much time has Dad got for his shoulder to heal?

Humping luggage on public transport may be harder than having luggage loaded into the car boot?

It's interesting to see a comment recommending "switching" to an automatic transmission. I have no problem with renting a manual when in continental Europe, and I have no problem driving on the left in the UK and Ireland. But combining the two would not be feasible. Shifting with the left hand is something I've never done in my life, and I'm not starting now.

RHD manual is not daunting for LHD drivers as the pedal and gear arrangement are the same (left-foot for the clutch, first gear to the left and fwd). The only slightly tricky bit is signaling a turn or lane-change whilst downshifting, as the turn signal stalk remains on the left of the steering column thus needs done by the same hand.

RHD manual is not daunting for LHD drivers

Just like German S-Bahn, U-Bahn, X-Bahn etc aren't confusing to Americans. Got it. I'm obviously a brain-damaged buffoon.

An update to buses to Castle Howard- from today (31 May) there is also a new Sunday bus from York Railway Station to Castle Howard (where it terminates, doesn't extend to Malton)- the X81-
From York at 0900, 1115 and 1415, from Castle Howard at 1005, 1305, and 1535

For anyone coming from further north Moorsbus have their M5 bus from Helmsley (and further afield, including Thirsk Railway Station and Market Place) on a Summer Saturday and Sunday- https://www.moorsbus.org/

The Moorsbus via Thirsk is an excellent scenic circular day out.