Car seat regulations vary from country to country. You should always check the road safety rules in effect in the country you are visiting. Your child's safety is at stake.
In the United Kingdom, any child under 135 cm or 12 years of age must be placed in a suitable seat.
If your seat meets this standard, bring it along! It's free on the plane and your child will be comfortable, just like at home.
Otherwise, you should consider renting a child seat with your United Kingdom rental RV. This option is payable in advance or on the spot, depending on the rental company.
Note that the seats in the back of RV rentals often have lap belts only (no shoulder belt). Most RVs have two seats with anchors for securing child's car seats.
Please note that not all rental companies have this rental option available.