We took Thomson's car hire in July and chose not to upgrade the car as the car they had compared the hire car to was a Vauxhall Astra which should've been fine for my party but when we arrived, Alamo at Sanford Airport tried to give me a Fiat 500. There was no way I would have even fit my sons stroller in the boot never mind the suitcases so I was left with no choice but to upgrade. This cost me almost $400 which would've been much cheaper to do in this country or even to take out a whole new hire direct with Alamo.
I would recommend sourcing your own hire car rather than going through Thomson's, when we booked car hire only through Virgin last year this was all inclusive, included a sat nav and worked out cheaper than what we paid this year.