Travel expenses: travel for necessary attendance: definitions: permanent workplace: regular attendance
You should check the other guidance available on GOV.UK from HMRC as Brexit updates to those pages are being prioritised before manuals.
Section 339(2) ITEPA 2003
An employee regularly attends a particular workplace if the attendance is frequent, or it follows a pattern, or if the place is one at which the employee usually attends for all or almost all of the period for which he or she holds, or is likely to hold, that employment.
It is reasonable to class anything done repeatedly, with some sort of consistency, that is, frequent or habitual, or follows a pattern, as “regular” attendance. This means that fortnightly travel, for example, is capable of being regarded as “regular”. However, it is possible that the attendance might be for a temporary purpose. The longer the interval between each visit, the higher the possibility that the purpose of the visit is a temporary one, therefore each set of circumstances will need to be considered on its own merits.