Wouldn't bother with Citizens Advice, well meaning organisation, but woefully under resourced.
If someone can't obtain legal insurance, (although standalone policies can be found I'm sure), the best options would be:
1) Most solicitors offer a free telephone consultation for 20-30 mins to give general advice, (or even just a bit of reassurance, or a touch of reality) which usually is unbiased and not trying to win them work. Be totally honest with them;
2) A solicitor will probably write a letter protesting your innocence/ requesting evidence on your behalf for about £50 if you want to show an operator you are serious.
3) If it does progress, always avail yourself of the free legal representation (available to all, regardless of financial circumstances) for any interview under caution;
4) Can always try pro bono outfits -
https://www.lawworks.org.uk/ to see whether you can get some free legal representation, although they make the same recommendation as me:
5) Worst case, represent yourself, but take a
McKenzie friend with you, e.g. someone with a bit of legal knowledge or experience in court.
Just because one home / car insurance policy doesn't include legal expenses (or not a sufficient legal expenses) enhancement, doesn't mean they all do, so shop around!
Phone or written legal consultation is basic, but would (should) be sufficient for handling initial communications with a rail operator (and again, essentially giving you some hope or a dose of reality.
Everyone CAN be a union member if they want to be. Nothing stopping people from joining a union, even if others in their company are not a member. Plenty of possible unions for office staff! My point from earlier is that your colleagues not part of a union (and with insufficient insurance coverage) will be more than likely absolutely and unexpectedly stuffed if they ever face criminal legal action!