I looked this up a few days ago, and the operating definition, by GP's in the UK seems to be;
Our definition of ‘Housebound‘ is:
A patient is housebound if they are unable to leave their home at all, or if they require significant assistance to leave the house due to illness, frailty, surgery, disability, mental ill health, or nearing end of life. For these reasons a person who is housebound would be unable to receive their healthcare in a GP practice or Clinic.
Which still leaves me none the wiser but as it uses 'at all', whereas I can, under normal circumstances, leave occasionally, for specific reason, provided the destination isn't too far from a bus stop, with nothing delayed, and in general everything goes perfectly (yeah right), then I can, sometimes, do stuff outside, almost always with payback.
This would suggest that according to the above definition, as they don't acknowledge PEM, and one step outside my front door (which is 30 foot up a stairwell) breaks the 'at all' clause - that GPs in the UK would not consider me housebound - and this is my experience.
The fact remains that I can very rarely go out, that I do not go out for social or recreational reasons, that even on the very, very ,very rare occasions that I do I plan so a trip from my front door to the door of my room in a hotel in Newcastle is under 1000 steps (over a day) - and I almost always get payback, even with a trip to my electricity meter or even the bins downstairs.
I still haven't got back to where I was before my last 'social' trip out, over a year ago, mainly because things didn't go perfectly on the way back. Things rarely go perfectly - which is another thing that people don't take into account.
I am 'quite limited' in what I can do, distances and time outside - and in general if it wasn't for critical need then I probably wouldn't go out at all. Far too limited to be able to justify it for recreation given the payback, and its impact on my ability to maintain the basics, these days.