I understand the experiencing different cars aspect and it would be an awesome 25 days but £12k and having nothing to show for it would seriously put me off this.
Its like going on a 12k **** up - I am sure it would be off the wall amazing but when I woke up with that hangover and a 12K bill (and possibly a tiger in the toilet) I would be seriously hacked off!
If you are going to spend a 100k on a car I am pretty sure test drives would be offered by any car maker listed above - decide which one you list best for free by letting dealerships give you test drives.
P.s. Sarnie- I hear your the man to talk to about mortgages. May need some advice in a few months time