It is valid for one return journey. Here are the detailed rules for the ticket you purchased:
http://www.brfares.com/#faredetail?orig=BTN&dest=0786&tkt=SVR
Here is a forum thread where it is claimed that U1256
means a single connecting Underground journey in zones 1-6, whereas a ticket with an attached Travelcard would be issued as London zones 1-6
.
Presumably what this means technically is that your ticket works as a single Underground ticket. Arriving at London Bridge you will go out through the NR gateline and then enter the Underground through its gateline. The ticket will stop being valid once you exit the Underground, except if you’re making one of the out-of-station interchanges marked as valid for “magnetic” tickets.
For LU-to-LU interchanges it seems that this means only Bank/Monument, Wood Lane/White City, Hammersmith/Hammersmith, Euston Square/Euston, and internally between LU gatelines at Paddington and Kings Cross.
Credit:stackoverflow.com‘