If you now intend to settle in UK, your reentry on a working holiday visa would be an illegal entry as the WH visa requires you to leave UK at the end of your stay as a WH

If the British Embassy in Belgium was prepared to process it, the best thing to do after your marriage there would be for you to apply for entry clearance on form VAF(1) as the non EEA dependant of an EEA National.

There is a good explanation on http://www.uklgig.org.uk/Civil%20Partne ... A_citizens

Although the website is particularly aimed at same sex partners/civil partners, the issues are the same.

One problem is that the British Embassy in Belgium may not be prepared to process the appplication unless you are resident in Belgium. It may be that you will have to go back to South Africa to make the VAF(1) application.

All very inconvenient, but if you succeeded in reentry after your marriage and then applied to stay on EEA2, 3 weeks later, there is a very high risk of you being considered an illegal entrant and that is obviously to be avoided as it could affect your future together.