Ring the dealer and tell them your intentions. Never had a problem in the past, so long as you appear to be serious about purchasing.
I usually explain about wanting to judge if a certain car will work with our lifestyle (i.e. I can get everything into it for our usual routines, etc).
Something you simply can't work out with a single 30-60 minute test drive.
I got a Discovery 4 (after they were called Discovery 4, but before Discovery 5) just for the asking. I went through all the usual stuff, looking round it, going through the numbers etc... Then said as it's obviously a big outlay, I would like to make sure it works for us. As it happened, I had a friends BBQ to go to at the weekend about 150 mile round trip, family in tow, overnight, so would be the perfect opportunity to assess how well it would work for us. As we regularly do this sort of thing, or fire off camping for the weekend or whatever. The sales guy spoke to his manager, and they gave us one of their business managers or whoevers car for the weekend. Sure, it wasn't exact spec or anything, but it was enough.
Did you order one?
Probably not - which is why extended test drives are really very difficult to get. They are generally only offered on new cars and only if the salesman believes there is a high prospect you'll order a car. Most people don't ask for or need an extended test drive before buying a car, especially with higher end kit where many will buy without test driving at all.
The test pilot:buyer ratio on extended test drives must be through the roof.
You wanted to change a 50k Range Rover for a 55k disco?
No. I had a Range Rover at the time (albeit an ancient P38), but it was my £50k 530d I had at the time (rrp), that I was looking to change for the Disco as the 5 series was getting too small for my needs.
In the end, I bought the Alhambra to use as a third car, just to see if I really did need the extra space as the 5 series was a great car. But then ended up swapping the 530d for my more fun Edition 30 as the Alhambra fulfilled the daily duties better, for me, and the 530d wasn't getting used enough to keep it (~5-8k pa). Of course, now the Edition 30 gets much more use than the 5 series was getting (10-12k), but I still do about 15k in the Alhambra per year.
So just so everybody is clear;
* You considered a brand new £55k Discovery and purchased a 10 year old Alhambra
* You considered an Aston Martin and purchased a 10 year old Golf
What are you considering next?