In certain circumstances I am exempt from using baby / booster seats............

But Yes Andy, it was a pain when Miss Vamps was little and a parent may take 2 or 3 to an activity and another parent pick them up, we used to have to call round to swap them over..........

Not just you, EVERYBODY who has an unexpected journey:-
2. When a child can travel without a car seatA child can travel without a child car seat in some circumstances.
Taxis and minicabs (private hire vehicles)
In a licensed taxi or minicab:
children under 3 years of age can travel without a child’s car seat or seat belt, but only on the back seat
children aged 3 years or older can travel without a child’s car seat if they wear an adult seat belt
Unexpected journeys
If the correct child seat isn’t available, a child over 3 years of age can use an adult seat belt if the journey is all of the following:
unexpected
necessary
over a short distanceYou can’t take children under 3 in a vehicle without a seat belt or the correct child car seat (except a taxi or minicab).
No room for a third child seatChildren under 3 must be in a child car seat. If there’s no room for a third child seat in the back of the vehicle, the child must travel in the front seat with the correct child seat.
Children over 3 years can sit in the back using an adult belt.
Vehicles without seat beltsChildren under 3 must be in a child car seat. If there’s no seat belt, they can’t travel.
A child over 3 can travel in a back seat without a car seat and without a seat belt if the vehicle doesn’t have one.
....another stupid bit of legislation. You drop your kids off at swimming/football/etc & you can't get back for what ever reason. You ask your mate if he/she will pick them up for you ...... no booster seat - unless Little Johnny always carries his booster seat with him...
So reading the above if you can't get back 'for whatever reason' it becomes an 'unexpected journey', it isn't a stupid bit of legislation, its actually quite sensible in the way its written. If you knew you were not coming back then theres no reason why you couldn't have left the seat with whoever maybe collecting the child
Guess its much more fun to bitch and moan whithout knowledge