A friend of mine who was a dispatcher on Ryanair flights out of a London airport says this commuting is extremely common. Sometimes as common as every other flight, he would have random flight deck crew, always in uniform, come to the departure gate and ask to be put on the loadsheet. They would not count in the TOB in pax paid heads, but they would be in the overall THOB count.
However, they never had any ticketed documentation, only uniform and their airline ID/airside pass.
In order to get airside they therefore went through a staff search channel. This is expressly prohibited at all UK airports unless you are actually on duty and working. These guys commuting are NOT working and are NOT on duty. This is therefore a breach of the DfT rules, unless there has been a discreet agreement made in the background.
An airside pass remains the property of the airport that issued it, can be revoked at any time and does NOT confer rights of access.
Airside IDs are only for use when on duty. These commuters are not on duty, else their commuting trips would count under FTL and they would also be on a ticket (whether full pax ticket or issued by airline for commuting purposes).
If any airport security officer decided to be clever one day, they could well ask these commuting staff if they can prove they are actually on duty and working. They would then be caught out...