I think for your mentioned cars there would be a doubling if not more in price even up to a factor of 10 I suspect.
All depends on the condition.
There are several Grabers out there, ranging from in-restoration to restored. You're potentially looking at £30-80k, depending on condition
The P6 convertible is probably worth around the £30-40k mark. Older restoration, but has had work completed over the years
T4 is probably valued at around £250k
The P6 racer is around £100-150k
As with any car, it depends on your point of view. Some people like having cars that stand out, others prefer the subtle approach.
Either way, your average P6 owner really isn't the intended audience for an expensive P6.