That is fine. Byelaw 18 states:
"(3) No person shall be in breach of Byelaw 18(1) or 18(2) if:
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(i) there were no facilities in working order for the issue or
validation of any ticket at the time when, and the station where,
he began his journey; or
(ii) there was a notice at the station where he began his journey
permitting journeys to be started without a valid ticket; or
(iii) an authorised person gave him permission to travel without a
valid ticket. "
"authorised person means:
(i) a person acting in the course of his duties who:
(a) is an employee or agent of an Operator, or
(b) any other person authorised by an Operator, or
(ii) any constable, acting in the execution of his duties upon or in
connection with the railway; "
Clearly the booking clerk and the guard act as such. In such cases, I would always suggest asking the guard before boarding, as:
a) you are clearly showing intent to pay
b) the guard knows that he has given you permission