Hire through enterprise. Book smallest care they have – once you get there ask how much to upgrade to a bigger car. It has always been £5/day more to upgrade to a zafira.
Dunno if this works at all locations, and it doesn’t work every time, but has worked for me a few times:
Book your car from Enterprise on the website the night before you need it (after the branch has closed for the night). Book something small. There’s a good chance that when you turn up (at opening time) the next morning it won’t be there – the computer said there was one available today because there’s one due back today, but you’re here at opening time, it isn’t back yet, you need your car and you need it now – free upgrade to something that they do have in the lot.
For bonus points, if it’s a location where you can see in through the fence, cruise by after closing time and see what they’ve got in – book something that you don’t see that size of.
Best result I’ve had off this – booked an Astra, got a Navarra big 4×4 double cab pickup.
If it all goes wrong and they’ve actually got the car you booked, proceed with glupton’s approach above.