This may seem obvious but after reading the Prosecutions threads I want to be sure of my ground. I have an Anytime Day Single, Bishops Stortford to London Terminals which is marked "Valid via any permitted route". As I want to go to Euston can I take the Victoria line from Tottenham Hale to save me having to go all the way into Liverpool St and then use the Circle/Met to Euston Square or is this ticket only for Liverpool St. Many thanks in advance...
National Rail tickets are only valid on the London Underground for the purposes of cross-London transfer (where permitted, this is normally indicated by a Maltese Cross ✠), or where the route in question is designated an '
interavailable route', or where they are specifically issued to one or more London Underground Zones.
If you hold a ticket to London Terminals then there is obviously no cross-London transfer included. And whilst there are some interavailable routes involving the Victoria Line, none would apply to this journey.
So if you want to take the Victoria Line from Tottenham Hale to Euston, you could either buy a
Bishops Stortford (BIS) to Zone U123* ticket, or split at Tottenham Hale and buy a
BIS to Tottenham Hale (TOM) ticket, plus a
TOM to Zone U123* ticket - or simply use Oyster/contactless for the Underground.
However, you mention Euston - are you actually making a journey to further afield? If so, this may change which option is cheapest.