The centre (which doesn't really show up on Goog maps) goes anti-clockwise, bear in mind that we drive on the left and go clockwise around a roundabout usually. You can use the outside to avoid the centre completely, which is often faster. The main problem is that it is very heavily used and hence some of the markings are scrubbed out. Also it is easy to get disorientated but even if you fly out of the wrong exit, it is easy to use a side street later to get back on track.
I have to say that I would not recommend visiting Swindon to someone who usually drives on the right hand side. To be fair I don't really recommend Swindon to anyone 8) For me, the MR is the exciting end to a two hour drive to visit a customer - just what you need to wake you up at ~0930.