There are two seperate problems with alternative wheels for the P6. As you rightly observe the first is the stud pattern. Five stud 127mm pcd is not common, in this country the only other users I'm aware of who had smart wheels were the SD1, Jensen Interceptor and Reliant Scimitar. Older Jeeps and numbers of other American cars also used this pcd.
The more difficult problem is a double whammy thanks to the P's De Dion rear end and the age of it's design (frozen at least by 1962). The de dion comes with a diff fixed to the base unit, so the drive shafts necessarily swing through an angle as the wheels go up and down relative to the diff. With simple universal joints this poses a problem in that at high angles UJ's impart a jerky motion to the driven flange. That's why modern cars have Constant Velocity joints on their front driveshafts and not UJ's. For this reason Rover wanted to keep the driveshafts as long as possible to minimise the change of angle. In a narrow car, as the Rover is by today's standards, this meant that they pushed the hubs out as far as possible and then used a very heavily dished wheel to bring the tyres back under the body. So the ideal ET, or offset, to bring the tyre central in the wheel arch is 53. This is, to put it mildly, unusual and very few oe wheels come near. It's also pretty unusual to find aftermarket wheels that heavily dished. This wouldn't matter too much - you'd just have the replacement wheels projecting rather further out of the body than the originals - if it weren't for the design of the rear wheel arch. Remember the date of design freeze - Rover didn't anticipate wide wheels then - so there is a pinch point at the front of the rear arch where the arch wraps over the front of the tyre and is also fairly close at the back. So there is no scope to depart from the ideal ET unless you are prepared to undertake very substantial bodywork. Worse the affected part of the rear arch carries the rubber door seal for the back doors. So if you do go for body mods you risk having to alter the door as well. Small relief can be gained by keeping your road springs fresh and the ride height high - this helps keep the tyre away from the affected area - and also by removing the double skin section from the rear of the affected area. Upshot is that the maximum tyre width the original arch can accomodate is 205, which removes some of the point of going for big wheels in the first place. Note that you might think on inspection that it would take slightly more, but you need to allow a little more room each side than on a normal car because the de dion causes the rear track to move in and out as the suspension works. Note also that it is not a good idea to go for a very large diameter wheel and then fit very low profile tyres - the juddery effect imparted by the driveshaft UJ's is absorbed to some extent by the depth of the tyre side wall. (This applies to modern BMW's as well unless they have CV joints instead of UJ's!)
So that's the ground rules. Experience suggests the following wheels as suitable: P5B 15" Rostyles but with max tyre size 195 (the offset isn't right so tyre size is compromised and anyway they are not very wide so a 205 is too big for them); SD1 14" alloys (two varieties, early V8 S which I think looks really nice or later VDP) max tyre size unknown, but likely to be either 195 or 205; SD1 Vitesse 15", which will take a 205/65 if you skim around 3mm from the hub mating face.
If you're after best tyre size then the 15" Vitesse is by far the most satisfactory, but it's not very light and quality control is poor - the ET varies between wheels, sometimes even on the same car(!) so you have to measure the wheels before skimming to get them dead on.
There were alloys made for the P6 contemporarily. Cosmics did one as did Minilite's and I've also heard of Commotive Super Slots (not sure if that's exactly the right description) and these sometimes turn up on Ebay or on vehicles. You can still buy the Minilites as the firm is still in busines! These are generally 14" and not very wide - the 205 restriction still applies. Rover offerd as options a 14" version of the Rostyle, Minilites and in the USA Magstars.
Note that adptor spacers to convert the stud pattern are not an option as they make a bad situation with the offset even worse. If you wanted to change the stud pattern you would need to remachine the hubs after plugging the existing stud holes(!!). If that was done there are a number of modern cars out there which might conceivably be donors - not least the Porsche 911 family!
fitted with 205/60 R 15 tyres.