You can try Motoward in Surbiton http://www.motoward.co.uk/
for jackets, pants, boots and armour.
Bracken had a few good specials on clothing a while back (sorry, I don't have their address, but they are in SW London).
After spending a long while humming and haa-ing over boots, I narrowed it down to the SIDI couriers (Infinity) or the BMW's and went for B'eM's. They are incredibly comfortable and so far worth worth the extra cash I needed to fork out.
If you find a place with a good selection of helmets, let me know as I'll need to get another one before leaving.
I've been through the pain of the gear hunt as well, so if you would like to know anything more, email me.
If you're flying into SA, it may be worth buying some of your stuff out there, I got most of camping gear out there in Aug last year.
Also, have you booked your license already?
After doing my accelerated course with 2 companies last year, the difference in instruction between the 2 was incredible and I will gladly recommend the second if you like.
I failed twice with the first company and after redoing my course with the new company, passed without a single mark on my sheet.