I can see your issue especially as you’ve moved from somewhere where things are done differently. But this is all completely normal for the UK so you might as well get used to it!
The box rooms thing is infuriating but people have and do use those rooms as small bedrooms. Not at all ideal, but houses are smaller here. Never seen one advertised as 2+1. It means if you want three big bedrooms you usually have to look for 4 bed properties.
The long thin gardens - well, loads of people do extend into them. There are lots of conservatories because they didn’t require as much planning permission and were a lot cheaper. They’re less popular now. And people here (in general) love having gardens, and don’t see them as “unusable” for most of the year - one key thing is they increase the distance from your neighbours! And for the part of the year when you can play out in them, the bigger the better. And yes lots were used for growing veg etc (and lots have been sold off and built on).
The bathroom thing - the key is “in my budget”. If you had a bigger budget you’d find the houses with less compromise. That’s what people who put in a 4th bedroom and not an extra bathroom did. They likely had to choose and didn’t have money or space for both. And had probably been sharing 2 kids in one room, one in the other, parents in the third, so moving to 4 bedrooms helped the sleeping space but as they weren’t adding more people to the house they didn’t have as much call to also adjust the bathroom space? It’s a cost and compromise thing.
Downstairs loos weren’t the norm and most people have had them put in so yes it’s a selling point.
Good luck with the hunt - and if you’re looking in London, the problem is pretty much always that everything is soo expensive you can’t get all that you want!