From ROSPA:
It is safer for your children to travel in the rear seats than the front, and if you can fit a child car seat into the centre seat, then this is the safest option.
The middle rear seat is the safest because it is the furthest away from the sides of the car. If it has a three-point (lap and diagonal) seat belt, this is the safest place to put a child restraint (unless the manufacturer's instructions say it fits better in one of the other seats).
However, it can sometimes be difficult to fit a child seat into the middle rear seat if there is only a lap belt, or because of the way the vehicle seat is moulded. If you cannot fit the child seat to the manufacturer’s instructions with little or no movement forward and back, or side to side, then one of the other rear seats is the safest.
The rear seat behind the front passenger will allow children to get out onto the pavement, rather than the road.
You should never put a rearward-facing baby seat in the front if there is an active passenger airbag. It is illegal to do so, as if the airbag goes off, it will hit the baby seat and fling it forward with considerable force.
If the car does not have airbags in the front, or if they can be deactivated, or if the car has sensors which does this automatically when you fit a child seat, then you can legally carry a rearward-facing baby seat in the front.
If you must fit a forward-facing seat in the front, make sure that the car seat is as far back as it will go so your child is as far away from the dashboard as possible. Double check that the child seat is very securely held by the seat belt and your child is securely held by the child seat's integral harness (if fitted).