Its not that easy to switch from us to uk layout for manufacturer if they didn't think of that in the start. They would need to design new pcb and wiring would be a bit different and so on. On the other hand Uk layout = ISO layout that most euro countries use with few different characters here and there. (new market new possibilities )
Oh and about those keycaps its not just put different plastic in machine and press them out. Transparent plastic has different cooling curves and stuff and all these things must be calculated or simulated before they start doing it. Other thing is you can press out one set and you are done.
The difficult part is the PCB and the key physical layout, but this is already done, given that the DE and FR layout are already in production.
Is just matter of putting the right file in the laser writer machine, and even that file is already prepared, the UK layout is available on the not backlit BW.
It's just matter of laziness and/or of (wrong) commercial policies