Atlas Cars have brilliant built in ones. We usually use KV but found their child seat situation a bit unreliable! So used Atlas last time and they were great and had the right one for our 3.5 yr old and our 1.5 yr old and same price as KVC.

we've used KV Cars and they provided a car seat.
Though we actually needed two, and they supplied one toddler and one baby which wasn't ideal.
Hopefully they will get it right if it is just the one you need!
Without sounding too bossy, if you are near Claphanm Junction then the train is by the far the easiest and quickest (and cheaper!) way to go

Which taxi companies can provide a toddler seat for a 12kg two year old? Need to get to Gatwick on Saturday and have only just realised will need a carseat.
Thanks