Exported 100's of cars over the past 15yrs, mostly to southern Ireland and more recently France (nich market stuff that I know where to place). It's the way I've always done it and never had an issue.
I suppose as long as the car is in the trade, you could just hand the V5 as if passing to another trader (you'll have their details from the invoice incase it all goes Pete Tong) but I've always filled in the export part. Although to be fair I usually arrange the shipping and export this side, and rarely meet any foreign buyers. All docs get sent on in the post.