This question has been highlighted in the European Court when a Spanish employee claimed that he should be paid from the time he left home until the time he arrived back.
The Working Times Directive does place a 48-hour maximum on the work time demands of an employer, upon their employees. The employee can opt out of the 48 hours but this is entirely voluntary and the employer cannot insist upon it, either for existing staff or during recruitment. Another factor, affected by classifying journey time as working time, is the period between breaks and the duration and number of breaks allowed.
The engineer pay and benefits survey, conducted by the FLTA annually, is due to be circulated in June 2020 and I will see if we can get a consensus on this subject from the respondents.
With employees having most of the rights under law, it may be that employers are ultimately obliged to pay engineers, without a normal or habitual place of work, from the time of departure from their home, to the time of their return.
The article below gives you some additional detail on this matter: