It should be as I believe Baker Street is one of the stations listed where you can make use of the underground for 1 journey in either direction.
I can't find the list right now.
I know Baker Street / Marylebone is on the list, which includes stations as far as Amersham. My question is that because Marylebone isn't on a valid route for the National Rail journey between West Hampstead or West Hampstead Thameslink and Brighton, is it theoretically allowable to start the tube journey at an interchange station which is not "appropriate to the route of the through journey being made".
National Rail Enquiries said:
Tickets displaying this sign are valid for travel between any 2 stations shown in the list below,
appropriate to the route of the through journey being made.
(skipped)
- Aldgate
- Amersham
- Baker Street
- Balham
- Bank
(skipped)
However in this case it is a non-problem because the tube fare from Rickmansworth to Marylebone or any tube station in Zone 1 is the same, therefore the tube ticket / PAYG can be used all the way to Victoria where a legal break of journey can be used to start the paper ticket portion to Brighton.
Whilst travelling via West Hampstead may have avoided zone 1 Rickmansworth to Marylebone is preferable due to the slow Met trains. Interestingly ticket failed to open gateline at Victoria and Brighton
It opened the Gatwick Express gate at Victoria in the past, but after a recent update it no longer does so. It has never opened the Southern gate at Victoria, however it always open the gate at Brighton (but not at Hove) successfully. I suspect that the Southern barrier aren't configured to accept the CBA / CBB ticket codes for these tickets, as they are mainly only available on Thameslink network.
I am a frequent user of this ticket (with 26-30 Railcard discount).