It's nigh on impossible to get a Nissan for a weekend, let alone a Ferrari 599 to a customer they don't even know
Must just be your shining personality then
One other thing to note is that most of these supercar clubs come with a very limited mileage policy when taking delivery of the cars, so you might not be able to put the miles on and do the exciting trips you want without a substantial further cost. That said, the same applies to regular supercar hire so even that route wouldn't be any better.
One further thing to consider is how they decide who gets what car. It's not inconceivable that you could end up with this list for a year:
Audi TTS
Bentley GT Convertible
M3 Coupe
Gallardo
G37
Cayenne GTS
Cayenne S
RR Sport Supercharged HST
RR Sport TDV6 HSE
Now to me that's definitely not worth the £12K per year, so I would definitely contact current members of the club both old and new and ask them what they wanted and what they actually got before committing to anything.