I would check who exactly you are expecting to be working for and in what role. London Underground do not use agency staff to cover operational roles, although third party recruitment firms are used for high-volume campaigns, but the successful candidates would not be employed as agency staff but would be employed directly by London Underground Ltd.
If the job is with TfL, then I would check exactly what you are expected to be doing, TfL do use agency staff to do things such as staff the call centres but these are not what I would describe as "railway" jobs. It is also possible that you could be working on behalf of a third party operating services on behalf of TfL.