Contact your carrier to verify that there are no restrictions, blocks, or filters on text messages.
Restrictions, blocks, or filters on text messaging will prevent iMessage and FaceTime registration from completing.
If "Waiting for Activation" still appears after you perform these steps, turn FaceTime and iMessage off and on in Settings > Messages and Settings > FaceTime. You may need to wait up to 24 hours to complete activation.
Most attempts will take less than an hour, but some attempts may take up to 24 hours. If your issue isn't resolved after 24 hours, please contact Apple for assistance.
If you've recently performed a remote wipe on your iPhone, wait 24 hours from the time you performed the remote wipe, and then turn iMessage off then back on.
So, I suspect the international sms messages goes out, but, Telkom Mobile does not have return paths for some international carriers.
Telkom Mobile does not have agreements for return international sms'se with all carriers. They only support certain carriers in certain countries.
FAQ:
http://www.telkommobile.co.za/help/international-sms
Actual supported operators:
http://www.telkommobile.co.za/media/heita/web/pdf/International_SMS_Countries_Operators.pdf
I honestly think this is the problem...
Oops, I do believe you're right. Now to track down which carrier is being used by Apple in the UK...
It seems there is simple solution to this problem. If you can activate phone using fresh (non-ported) prepaid TM SIM with some airtime, problem is solved. After activation is completed, swap the SIM and use phone normally.