https://www.caa.co.uk/Passengers/On-...ng-allocation/
Child seats and child restraint devices
Infants under the age of two need to be secured whenever the seat belt sign is on. This can be achieved through the use of a seat belt loop provided by the airline to secure the infant on an adult’s lap, or by using a child restraint device i.e. car type seat, or an alternative provided by the airline.
Forwards facing child restraint devices may be installed on both forward and rearward-facing passenger seats but only when fitted in the same direction as the passenger seat on which it is positioned. Rearward-facing child restraint devices can only be fitted to forward-facing passenger seats.
Child restraint devices can only be fitted on a suitable aircraft seat using the type of seat belt/harness for which they were designed. For example a child restraint device that needs to be secured by a car type seat belt which includes a shoulder harness, cannot be installed on an aircraft seat that is fitted with a lap belt only.
The permitted use of restraint devices can differ between airlines, so it is important to contact your airline in advance of your booking or flight to ensure that the device that you may wish to use is acceptable.
I don't know if this clarifies things, as the last time I had any interaction with a child seat would have been when I was in one too!