The cost of attending meetings double handed far exceeds the difference between 1st and 2nd class travel! A technical Project Manager in what was MoD(PE) was required to be able to carry out every job in the team, so there was seldom justification for anyone else to travel, the exception being if he had a trainee with him.
When IPTs were formed (2nd time round, not the late 80s model) IPTLs were given the authority to drive a bus through the regulations. It was "their" budget and they could spend it as they liked. So you'd get the indirect labour (invariably the senior grades/ranks with no responsibility or accountability) swanning off on 1st class jollies and drinking the hotel bar dry on the IPT account (literally, on one notorious occasion) and direct labour (usually the lower ranks, but those with the signature) refused permission to attend the likes of design and safety meetings they were meant to be chairing. That would explain the situation Dalek describes.
This also serves to highlight the basic problem I've come across in many IPTs . The "pyramid" is inverted, with the most senior staffs surplus to requirements as they contribute little. And yes, before you ask, I've been an IPTL, but 23 years ago when IPTs were not bloated with untrained staffs each doing a fraction of a job!