Yeah, I imagine 6 foot wheels would rub!

12x6 will rub. Even 10x6 will rub. It's just a minor bit or trimming and paint touch up on the edges. Sounds like more of an issue than it is. Go for the wheels that you like best. 

I like to put 12x6 rims not 13

Got rid of the 6' wheels in a couple of weeks, yes they rub.

They'll also catch the inner wings at the front on full lock.  The factory effectively fitted Mk1 racks to the Sportspack cars to prevent this with the big 13" wheels.

Yes, tyres will rub the body panels.

Tyre size will the same as you are using currently. The additional 1" wheel width and the change in offset used to give these wheels a "deep dish" look means that a larger proportion of the wheel protrudes past the panel work. This means a wider sweep area for the wheel/tyre causing it foul roughly in the area where the front wing and front panel join.

The rims shouldn't rub. The tires might. You did not mention what tire size you plan to put onto the 6" rims.
You first have to look at what clearance you have now with the 5" rims and tires.
Beginning with the front:
You need to determine if the ride height is sufficient. Too low a suspension setting or worn knuckle joints increase the chance of rubbing.  Assuming the same tire, the width of the tread would be unchanged (where it is most likely to rub) and the width at the sidewall might be 1/2" or so wider. Next you have to determine the offset of the existing 5" versus the new 6". A greater offset will move the edges of the tread farther out so when you are in a turn, the angle of the wheel will put the tread closer to the wheel arch. Insufficient offset may result in rubbing on the inside somewhere.
At the rear wheels,  again check to see what your existing clearance is. Offset is the main factor. Insufficient offset may result in the tire rubbing on the damper (shock absorber). Greater offset may result in the tire rubbing on the arch at full suspension compression (going over a bump).

Hello I have a 95 mini  5x12 wheels with A539 tires With factory arches and they fit perfect I like to exchange them for  6x12 mamba Wheels some tell me they will rub in the front and Others tell me they don't