Sounds like a lot of shipping.
"Challenges" I would expect to be: price of a decent, rust free, unmolested hilux in the uk likely to be quite high, kitting out in uk with lots of overland kit which is probably made in S Af anyway makes little sense, buying unseen in the uk could leave you with a lot of work to do once/if the car drives off the boat in Cape Town. However, if you happen to be living in the uk when you buy, the only advantage I can see for this course of action is that you know what you've got before the whole thing starts.
The practical challenges of shipping etc are all covered somewhere on this site, and everything's possible, but if the plan is to sell the car on, I think you're overcomplicating things and going to needless expense by buying in the uk.
Good luck whatever you decide,