There are a number of BTN seasons at £88.30/wk:
London Bridge FCC Only
London Terminals FCC Only
Farringdon Not Underground
St Pancras Not Underground
Blackfriars Any Permitted
City Thameslink Any Permitted
All of these are routeing points, members of the London RP group, with the exception of Farringdon, which is an associated station.
The mapped route for BTN - London is map LB, which includes Brighton - Victoria, Brighton - Waterloo and Brighton - St Pancras (via London Bridge, Blackfriars, and Farringdon).
Since Farringdon is not a routeing point, you should be able to choose any mapped route to a member of the London routeing group (except St Pancras, since you would pass through the destination on your ticket), and make your way from there to Farringdon by any means other than underground.
So I conclude that London - Farringdon Not Underground is valid to Victoria and Waterloo (and also to Clapham Junction) as a mapped route to London.
Note that map LB erroneously marks Farringdon as a London Routeing Point member - it isn't, and for this reason you shouldn't be restricted to travelling there via London Bridge.
To St Pancras, Blackfriars, and City Thameslink, Victoria, Clapham Junction and Waterloo are not mapped. The tickets may still be valid to these stations under the shortest route rule, but I'm not going to check, as the Farringdon ticket is valid as a mapped route.
Note: this is theory, practice may be different!