The field agent's job is more about having good people skills and the ability to appear trustworthy and instill confidence in the clienbts rather than any genealogical or research / record agent training or experience.
Most of the field agents are ex-policemen or ex-military, which also gives them integrity and hopefully an honest background.
The office staff are trained as record agents using the GRO index, the census returns and sometimes probate records (wills). the staff are salaried and with their training and not being sidetracked like an amateur family historian would be, they are able to home in on their target quickley and efficiently.
I have noticed they tend to use the old microfilm index rather than the online index, but foir the work they do, which is to find people by surname first, that is a far quicker way than the website format like ancestry.
To get the field agents job I think you would need to be in the right place at the right time with the right background and those agents also only seem to work on a part-time basis, as of course the lists are only published on a thursday and the contact has to be rapid.