Really, you're on holiday so it's not a car you have to live with, you could get a luxo-barge or SUV and waft your way down PCH but every minute you'll be wishing you were doing it in a convertible Mustang with the top down.
The hire car fleet are all pretty new, so the A/C is damn effective and you'll have no problem with temperatures with the roof up, and all the way through coastal California the air is plenty cool enough even when it's baking sunshine.
However the luggage space is somewhat small - not tiny but if you and your missus are planning on packing to the limits of the hold-luggage specs then you'll have to accept having one, if not both hold-alls on the back seat. You're stopping off at motels so most of the time it won't really be a problem other than aesthetically but it's something to bear in mind, if say you plan to stop off at Big Sur on the way up then you need to be content with having your luggage in the car 'on display' rather than in the boot.
Also, I drove a convertible (Sebring) from LA to LV in the first week of September and it was totally fine, yeah the Sun is baking so wear a hat to stop you getting bacon-tipped ears, and with the top up and the A/C blasting it's plenty comfortable.
Here's my suggestion - there is no car which is going to be perfect for all your requirements, so hire a roomy SUV/luxo car with plenty of boot space, good ICE and aircon from LAX-> San Fran. See if you can change to a convertible for the drive from San Fran to LAX.

If you can't then have a proper think about the luggage point and make a call between what is more important re: having a convertible.
After all, if you do PCH and love it in a SUV, why just come back again next year and do it in the other direction in a convertible
*edit*
Also 'handling', rapid acceleration, all the things that you'd look for in a UK car are really completely unnecessary for doing the kind of trip you are, you'll realise as soon as you get here that people are just way more chilled out (LA apart perhaps) and not in a rush to get everywhere like in the UK. And after all, the last thing you want to be doing is blazing along PCH focused on the next corner ahead, take it easy and enjoy the view, it's bloody brilliant - you can get your fill of twisty country lanes in the UK, they don't really exist in the US
