Matt, all transit accomm for London is handled by SO3 G4 at HQ LONDIST on Horseguards. The post is a held by a civilian who I used to deal with constantly during my time on the Specialist Recruiting Team. She is exceptionally helpful and being a civvy, she won't give you the usual guff you would normally get when dealing with HM's Mouthbreathers. Be aware that transit accomm, especially for ORs, is like rocking horse jobbies so there's a good chance she'll send you a non-availability chit for you to book HOTAC with - but please don't hold me to that! I'll PM her name to you.