Yes, through tickets that have London as a permitted route (as defined in the Routeing Guide) include a 'cross London transfer' on London Underground at no additional cost. These tickets usually (nearly always) display the cross-London "maltese cross" symbol, this indicates that the tickets will work the ticket gates on LU. I advise you to take the Victoria Line, then the Bakerloo, as the interchange at Oxford Circus is cross-platform. There is a trick to getting on the Victoria line easier at St Pancras but I am unsure what it is (some of the signs direct you a very long way round).
Please note: LON does not exist, please don't use abbreviations in topic titles (without the full names) and please only use abbreviations that exist. Correct 3-letter codes can be used in nearly all journey planners, ticket booking sites, the National Rail website, Avantix etc. Incorrect codes will leave people confused. Sorry if this sounds pedantic but we need this section of the forum, especially, to be as clear as possible for people to understand, thanks
