I'm currently in the process of applying to the University of London's External System and I have to mail them original copies of some official documents. Obviously I want to get the original copies back, so that means including prepaid postage from the Royal Mail. I've looked on their website and there is an option to prepay online, but you must use the postage within one day or it will expire. This just won't do. I see there are prepaid packages such as this one:

300g Worldwide Prepaid Package

But the problem is those packages would have to be shipped back and forth between the UK and Thailand a ridiculous number of times just to accomplish what should be a simple task.

The question is: how do I provide prepaid postage so that the university can return my precious original documents while based in Thailand?

Thanks for all your tips!

Do you have family in the UK, if so send the Docs to them and let them get them back from the Uni, then let your family send the stuff back to you after everything has been done.

weigh your packedge, before you seal and post it in Thailand put another envolpe in with the same post stamp already on for return.

Maybe international reply coupons are an option.