There are no import duties on bikes brought from overseas. You could expect to pay VAT (on the purchase value) and an Ad Valorem duty of approximately 7%.
If you get in touch with one of the international shipping companies, your bike can be shipped in a shared container. My RT came across before the container with furniture and so had to share a container. It cost me 620 pounds for shipping. This was through European Van Lines who I would strongly advise against using.
So, you could pick up a reasonable bike for say 3900 pounds. So, your landed cost (without registration and SA Handling fees) will be R 74718.00. The bike I've used as an example is a 2003/4 BMW R1150R. You can buy the same bike here in SA for between R59 and 65K.
When the exhange rate was R10/1pound, importing made a lot of sense, but right now, I'd say that this isn't the most logical step.
I've a dealer friend in St Albans (UK) who may sell he bike you buy for the tour, on a consignment basis, which means he doesn't buy the bike off you, but sells it on your behalf. You agree a price that you want for it, whatever he sells it for over that is his.
Bear in mind, though, Ebay has killed the 2nd hand car and bike market in the UK. Many dealers don't even take trade ins anymore (and an outright sell, you can forget about). People buy and sell on Ebay and consumer confidence is high. Bear this in mind if you buy a bike, this may be your best route to market.
Because of the competitive nature of Ebay, you're vying for the same seller that 20,000 other bike sellers are after. This has caused the "blue-book" values to become nothing more than academical in relation to the true value realised for the vehicle. Oh, what wondrous power the internet has!
Hope this info helps.
Lawrence