Upvote:4
Yes, this should be fine. U1256
really means "a connecting underground journey in zones 1-6", including 3 and 4, except it has been shortened to fit in the space on the ticket -- this is also the reason for the missing O in LONDN
.
Heathrow is in zone 6.
Beware that your ticket will not be valid on the TfL Rail or Heathrow Express trains to Paddington; you'll need to take the Piccadilly Line (and then probably walk from King's Cross St Pancras underground station; changing train to reach Euston by tube will be slower).