With respect, Mike, that's steering away from what I originally asked.
In my original post, I recognised that there are workarounds potentially available. However, I asked for help in how to install SQL Server with British English, since I don't want to put in workarounds until all alternatives have been exhausted.
Apart from the niggling doubts about little time bombs being introduced by installing with American English and using workarounds (unlikely but possible), there are practical considerations too. If I do install with American English, that means changing the default language and date format both for the instance and every login created as part of the installation process. That's an extra manual task that has to be remembered for every dev/test/sandpit instance related to the live instance in question for consistency, as well as the normal configuration checklist that has to be maintained - not inconsiderable in itself. My argument is that this ought to be inherently part of the choices one is presented with during the installation process - I'm sure MS wouldn't expect a DBA to have to install a German instance and then "just" change the default language.
I know this sounds like I'm trying to make something big out of what appears to be trivial, but the importance of having instances set up correctly and consistently with British date formats is real, especially when we're exchanging data with companies both in the UK and the US. Troubleshooting date-related import issues is niggly enough without having to remember to check the login has the correct defaults.
I guess if I can see MS has gone to the trouble of distinguishing between US and UK English as the instance language, I struggle to accept that MS are only providing one English alternative for installation. The answer could be as simple as me not getting the installation files from the correct download source, but I can't get MS to give an answer.
As a result, I'm afraid it's not "just" that I have to test our applications and make sure the logins are set to UK English, but sadly I'm running out of options.
Semper in excretia, sumus solum profundum variat