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.