Everyone's mileage seems to vary drastically on this topic. I think there are a few "golden truths", however.
- The brightening color change is never permanent; it is temporary.
- The process will make the plastic more brittle.
Some have decided to simply use bright hot sunlight and skip the bleaching chemicals entirely - I'm talking extreme sun, like in Arizona. Perhaps this is why you're considering bleaching since the sunlight and temps in Ireland are more subdued.
Even the use of plastic wrap can be tricky. Some people have great results, while others may produce marbling.
A friend of mine had severe marbling occur on his C128D keyboard shell. After that it was utterly ruined. So, you have to ask yourself if it is really worth it. If your computer parts look like English tea with milk that's one thing. If the yellowing is really minor that would actually make me second guess myself more, and probably just live with it.
Probably best to do a test run on something you're not attached to as you experiment with your environmental variables before going all-in on a precious piece. That's my advice anyway.