Thats is not in all cars here, its an option here still, but if you enquire with the hire company they should be able to give you ISOFIX on the hire car if you request it, but not sure if it is that same as the US system (as in the same distance between mounting points).
But if not you can pick up a seat in the UK for your time here, have a look on
www.mothercare.com they have some basic seats at resonable prices might save the hassle and risk of damage to your seat you use at home during transportation.