For some reason it's very hard to find window units for sale in the UK, single-duct portables are the only thing available for cheap (although it's a fairly easy mod to convert one to dual-duct).
Probably because the UK - similar to most of Europe - does not use the US vertically sliding sash window type, does it? The typical "walmart window AC" does just not really exist in (most of) Europe, because the windows for it don't exist, afaik.
Edit: Turns out, sash windows are more commonly found in the UK (compared to other European countries), but still not as common as in the US. So, UK = not as hot (so far), thus still probably not worth it (yet) as a market.
Why are the windows different, actually? They don't seem to be smaller overall, just skinnier and taller?
But you should still be able to get two tubes fitted into any kind of window with the right seals. If you were really up for renovations you could get closeable exhaust holes punched through your brick or something maybe.
Sash windows are just not as common. Seems like they are in the UK somewhat, though numbers I found vary, but overall in Europe they're pretty uncommon.
And yes, there are options for tubes/ducts for the more common window types. Like tilt-and-turn windows, horizontally sliding or all the other kinds of inward or outward opening windows - but most of them are the ducted portable units the original comment was speaking of, which aren't great.
There are also some better portable split units, but those are pricier and the install is not as easy. (They're great though.)
one issue with tilt and turn is getting window screens for them. It's possible but mostly they get installed on the inside if you have the proper wall spacing that allows window to function without interfering with the screen. mini splits are great but it's much more money than a simple window unit or portable unit.
Edit: Turns out, sash windows are more commonly found in the UK (compared to other European countries), but still not as common as in the US. So, UK = not as hot (so far), thus still probably not worth it (yet) as a market.