Hmmmm... well, I'm in the process of transfering the post-graduate work I've done towards a degree at a British institution. I do not think you are in the same position as me though. From what you've said, I think it may be best just to talk to your boss about it now, so he can make sure that all the proper paperwork is in place for you to receive your Masters. Also, this will get it off your shoulders and it will be out that you are in fact leaving. Hopefully that will save you from his wanting to re-evaluate the situation since you know you will be leaving in September. If you are set to get your PhD, I would think you can always, with your Masters, apply to a University in Wales. But, I think that because you said that he would need lots of advanced notice, I would tell him at the beginning of May, and then do whatever you needed to do to make sure you were done in September, not before or after. My 2 cents, and I hope it's helpful! Best of luck really! I know these are difficult situations!