The driving wheels supplied in the Class 5 kit appeared to be 16mm Romford / Jackson on extended plain-ended axles, nowadays Markits. I found them a bit too coarse for my liking, and replaced them with some plain 4' diameter Sharmans.
The pony and carrying wheels were plastic-centred (or at least the ones in my kit were) and I thought they looked like Kean Maygib. Being spoked, they're wrong anyway, as the real things had disc wheels with holes drilled around the perimeter. Modern equivalents could probably be Markits, but they wouldn't be cheap nowadays. The other alternative might be wheels supplied by the 3mm Society or maybe 3SMR, as they work to 12mm gauge anyway.
3mm Society list - see pages 18 and 19 for wheels
3SMR wheels price list