Agency work can be quite complicated and sometimes much worse than Zero Hour Contracts. (I've done both)
When you work for a company (Southeastern) you are directly employed and have a contract of employment, terms and conditions and in general its full time employment or time restricted (contracted hours etc) You also receive in work benefits. Not the discount schemes etc but things like Sick Pay, Holiday etc.
When you work for an Agency its different. You can be employed just like a Zero hour and basically be on call for any work that comes up. They often don't employ you directly. You are not much more than a name on their books they can call on. You have a particular set of skills and they know what your working restrictions are so can allocate you work any time and anywhere. With some Agencies you have more skilled workers and they are sector specific. In this case you would become one of those. You would be skilled and qualified in Gateline and could then be used at any Gateline at any time. Unfortunately you often have no benefits. They don't pay sickness, holiday etc and only pay you for the time you work. A bit like Zero hour but much more opportunity for employment.
Southeastern and other companies basically don't need to pay these in work benefits. They outsource the entire cost to an Agency and pay them a fee. They never have to deal with staffing issues and pretty much always have available staff. With no cost for NI, Sickness, SMP, Holiday etc they save a butt load of cash.
They also don't have a wage bill. Southeastern pays the Agency and the Agency pays you. Quite often, and is the case at Sevenoaks, the Agency pays you a different rate than the full time employees. Sometimes more, sometimes less. It often leads to bad blood between Agency and full time staff.
I've been Agency. I must admit I didn't like it but it was a means to an end. I was working and earning. My ex worked Agency and my Mum did for many years. There is an advantage for some people so I'd support it.