Cosmic did some spacers that had a male/female wheel nut type affair that was used to extend the studs. Avoid these like the plague they are lethal.
Some spacers use the existing wheel studs (obviously you fit longer ones). These are fine so long as they are a good fit and therefore support the studs over their length. That unfortunately means you are looking at specific stuff as the universal stuff is usually made for 12mm wheel studs and the Mini's are 3/8 (9.5mm ish).
There's a third type where the spacer bolts to the existing wheel studs and it has a second set of studs built into it. These are the best type assuming that the materials and manufacturing are good (the best design in the world is never going to work if you make it from rubbish material - ask the army about the SA-80 for a real world example of that). These are generally only used for quite large spacing as you need enough "meat" in the spacer to take it's own stud. They are often also used when fitting wheels from a different car as the second set of wheel studs can be a different configuration to the originals.
Personally I'd avoid spacers if possible - they are fine in principle and if used properly but the costs of getting good quality ones for a Mini is likely to be prohibitive.
Iain