It sounds devastating, really, really horrible BUT at least your buyer is still keen.
Our home was on the market before we came over. Then, at the last minute, we found a buyer and all the necessary paperwork was completed whilst we were here. It might not be ideal -and the guy did try to negotiate a discount when he realised we were emigrating (ha!)- but it worked just fine. And if the Crushes are right, maybe you'll have some of that 5% to help you settle.
Try not to stress (always easier said than done) and try to enjoy your last weeks in the UK.