First thing to check is your steering trim.

If the car is "crabbing" - i.e. it points forward, but moves to the side it's probably one of two things...

Are you testing it out on the carpet at home? If so, the weave in the carpet makes the car move to the side. It should run straight on smooth tarmac.

Otherwise, one of the suspension geometry settings is different on one side of the car to the other. It's hard to build a Mini wrong, since there is so little to adjust, one possible problem is uneven steering links and an off-centre servo saver. The other possibility is that something in the suspension is binding or misaligned.