Could anyone advise on which might be the 'safest' of the CMYK default settings to use for images intended for printing in a glossy magazine (UK) - I have no additional information to help me create a custom CMYK setting.
I'm intending to use either Adobe RGB or Prophoto RGB beforehand - any preference here?
Firstly you need to be aware that CMYK has less colours than RGB. so fluorescent greens and so on won't reproduce.
When you convert an RGB file to CMYK blacks maybe created by the use of a composit of CMYK (ie. 20%C 20%M 20%y 20%K) this can produce a muudy black. if you use 'selective colour' (image>adjustments>selective tool) tool you can dial out the CMY and just have blacks printed from the black drum. giving a cleaner end result. A good printer will do this for you.
I worked in pre press for 3 years, I'll try and upload a pic as an example for you.