Ubuntu/Linux version

I plan to move to the eu servers - the free account I set up shows image version...

Description: Ubuntu 16.04.5 LTS

It would be great if I could get that up to 18.04 before migrating? (gives me a few years before being deprecated). Also, when migrating I would have an immediate fall back if something breaks (though not really expecting that).

Right now our most recent system image is based on 16.04, but when we release a new system image it will be based on 18.04. Switching an account over to a new system image (or back again) is really quick to do on our side, so we'll be able to do that for your account when it's ready. The only risk is that point releases of Python might change (eg. 3.6.6 to 3.6.8), which can sometimes cause problems with virtualenvs requiring that they're rebuilt, and the specific versions of the pre-installed Python packages will change, so your code would have to be compatible with the newer versions if you use them.

Thanks Giles.

Is there even a loose time-scale or schedule for when to expect an 18.04 image?
(Knowing if it's weeks, months or potentially years will help in deciding whether to migrate now or hold off for a while)

It would probably be months, but no guarantees.