on Question Time, a section of their manual was read out. it was written in 2006. i don't have a verbatim quote but it said something about the use of prostitutes being discouraged.
If the actual manual for staff behaviour said that, there's no hope.
Essentially I'd think of that two ways - one, I'm surprised it mentions it all because I'd assume it was gross misconduct of the type that doesn't need to be listed.
If using prostitutes is not illegal (as it is not in the UK, nor in many countries) then why is it an employer's business if an employee does that at all? I would have no idea if any of my co-workers used prostitutes but if they did I would not expect them to be dismissed for it.
If it's illegal where they are then yes, that's a potentially dismissable action, depending on the contract terms.
The Oxfam issue seems to be a lot deeper though, there were underage 'prostitutes' (they're not really prostitutes if they are underage are they, just kids, being sold), porn on computers, sex on the Oxfam premises, threats to co worker witnesses and a massive cover-up. That is the issue.
Now, don't start suggesting I agree with prostitution, I don't but it is not illegal and, as such, nothing to do with your employer. I am really surprised an employee handbook mentioned it, it shows they knew there was an issue which is very worrying.