I found this on the childcarseats website:
Coaches and Minibuses
Coaches and minibuses carrying groups of children on organised trips, including school trips, must provide children with either a lap-belt or a three-point belt on a forward-facing seat.
All coaches and minibuses manufactured or first used on or after 1 October 2001 (whether they carry adult or child passengers) must by law be fitted with either three-point seat belts or lap belts on forward or rearward-facing seats.
However, seat belts are no use if they are not worn, so ensure that your child knows that he or she must wear their seat belt at all times, whether you are travelling with them or whether they are travelling on a school trip, for instance.
Seat belts are designed for adults, and the safest option would be for children to use a child seat that is suitable for their weight, when travelling by coach or minibus. Unfortunately, this is rarely possible because it is not practical for the Operator to carry a range of different child seats on their vehicles.
If you are travelling with a baby, keep the baby in a rearward facing baby seat and use the seat belt to secure it, if possible.
If an older child is travelling on an organised trip, ask the organisers about child seats in advance, although it is unlikely that they will be able to provide them. It may be possible to use your normal one if it can be fitted to a coach seat. However some coaches have lap belts and some have lap and diagonal belts, so check whether your child seat is compatible with the type of seat belt fitted on the vehicle. Discuss this with other parents so your child will not be the only one using a child seat.
In most cases, it is likely that the children will have to use the normal seat belt. Although not ideal, this is still far safer than not using any restraint. To maximise the protection provided by the seat belt:
the belt should be worn as tight as possible
the lap belt should go over the pelvic region, not the stomach
the diagonal strap should rest over the child's shoulder, not their neck.
Three-point seat belts (lap and diagonal) provide greater protection than lap belts. However, lap belts are far better than no belt at all.
Do not be tempted to put a seat belt around yourself and a child on your lap; in an impact, your weight would crush the child.