In the UK security knows what computer parts are and how they look on the screens. If in doubt they might ask you to unpack an item so they can actually see it.

You should bring these items in your hand luggage, as valuables and breakables should not be packed in your hold luggage. But you seem to have too much for your hand luggage. In that case you will have to pack some in your hold luggage and take the risk it goes missing or gets broken.

The UK side will only check whether you can do damage to the plane and the people in it and computer parts are not known to be dangerous.  

When you arrive at the other end you will have to declare, the customs in South Africa will value and charge you to what they think it is worth. It does not matter whether you still have the parts in boxes or home wrapped as they will judge by eye whether they are new or almost new and charge accordingly.

Never lie to immigration and customs officers, as they can make your life hard if you do, and trying to hide the fact that they are new is a kind of lying. Some people will be able to pull it off, the rest will be charged for new parts with possibly an extra charge for trying to deceive.  
As you will get your hold luggage before you go through customs, it does not matter in which part of your luggage you have your computer parts.

With a value of 2000 pound, I am pretty sure customs in South Africa will want to charge you.  
Only if you or your husband will bring them out soon, you might get away with a 'temporary import' although I am not sure how they handle that in South Africa.