Everything that can be said, has been said. Get a TRMS multimeter if you want to make sure the current and voltage are absolutely within their operating values. They're only 50 euro these days. Alternatively, borrow one.

the Folham workhorse is an electronic ballast often claimed to drive small SOX lamps correctly and is a rapid start floro ballast so a starter is NOT required and is available in 220-240 volts 50 HZ
maybe put a saved search on EBAY till one comes up and like stated above check current / voltage once SOX is lit

I wasn't successful in my past attempts to run SOX18 off of F20 ballasts. It wouldn't warm up or get bright and I imagine F20 and F18 are similar enough. That's why I said I was much more successful in using a 2 lamp F32T8 ballast wired to one SOX with both blue wires and that was perfect looking.

The problems i mainly had with 120V F20 preheat choke ballasts included not having the lamp striking at all with FS2 starters and having buzzing and failing ignitors when attempting to ignite the lamps using ignitors designed for low voltage high pressure sodium lamps.

However, when I tried using 277V F20 preheat choke ballasts in conjunction with FS4 starters and a 120V to 277V step up transformer, my 18W SOX lamps managed to maintain a stable arc and operate as intended. However, some FS4 fluorescent starters would not work while others struck the lamps without any issues except for a very long blinkfest with some starters.

the Folham workhorse is an electronic ballast often claimed to drive small SOX lamps correctly and is a rapid start floro ballast so a starter is NOT required and is available in 220-240 volts 50 HZ
maybe put a saved search on EBAY till one comes up and like stated above check current / voltage once SOX is lit

Yup, I believe there's a universal voltage and frequency version of the Fullham workhorse ballasts that runs directly off the mains in *most* countries. I think Japan being one major exception.

there are "WH UNV" series that are 120-277 and they start with a "4" in there number with the WH41 being recommended for a F20T12 single tube
and they offer there "normal" series in 240V configuration and being electronic are 50-60 HZ compatible

I'm looking at 18W sox ballast alternatives now and as far as I understand all you need is a ballast that gives similar enough current and voltage? plus a simple starter to get it going like an S10? I'm just trying to understand the criteria for it to work.

I have an old F15T8 magnetic ballast that's rated at 0.33A for a 15W tube, figured it might work.
If anyone has any tips for european (220-240V) ballasts that would be good. That can be bought on for example RS components.

https://youtu.be/1DfxKqt4CLY

A 15w ballast rated for 230v operation will work, slightly underdriving the lamp at 310mA instead of the lamp's rated 350mA. That is so close that i don't think it has much effect on the lamp's life span. You can also try and find an 18w one, see how that works. In the video above you can see how i hooked up a s10 starter and a capacitor to start the lamp, without having an actual "proper" SOX ballast and ignitor.
I still don't know yet how the starter-capacitor trick works - but it does, and works fine with 2 different, used 18w sox lamps.

The important part is the current. The current must be correct. The voltage will automatically get to the correct level if the current is right.

Side note: interesting ballast. I have never seen 25w lamps. 23w, yes. But 25? Nope.