We had the same situation when we went on holiday to the UK. We ended up driving straight from the airport to Tesco to pick up a booster seat as it was going to cost around 200 pounds and the booster was 10 pounds. DS was a lot older though and not too far from being out of the booster all together. The car seat hire is a wrought frankly. But I think different companies have different car seat policies. Now we live in Scotland and usually book our hires through Arnold Clark (we didn't bother buying a car). They only charge 1 pound per day for a baby seat. So my advice would be to look around the car hire companies to see the best deal once you add in the seat.