The main provider of the apprenticeship can provide apprentices with their ULN number and the main provider’s UKPRN number, you can then provide that to TfL to allow a discount.  This is a benefit for England based apprentices.

ICAS do not provide apprenticeships as a main provider anymore, so you would need to speak with your provider, (ie your firm, often EY).  So you should speak directly with the firm apprenticeship team to obtain the info.