My point really is why is this only happening now?
Has the buyer only just put their first mortgage offer in?
I still think a lot of the confusion here has come from you referring to valuations; which the buyer has no control over, versus their mortgage application.... and the estate agent has now followed your lead, presumably because they think you're worried that the valuation won't be enough.
My best guess would be that the buyer has put their full mortgage application in, and the bank is now going through the motions, which will include validating their deposit if it's from overseas. That can take some time, depending on where it is.
It's more than possible that the bank has been crazy, especially with the stamp duty holiday, and they've only just got round to this part. It's totally out of your buyers' hands, as is the valuation, because the bank will order and arrange that when they get to that part of the process.
Do you know if your buyers want a survey? They'd have control over the timing of that, although they will likely be advised by their solicitor to wait until they've got a mortgage offer to have that done.
It is a really frustrating process at times 
. Our last buyer was using Kensington, and they're even slower than mainstream banks - usually 6 - 8 weeks for an offer, but they were quoting 10 - 12 during July and August. Our mortgage with Barclays took 4 days from application to full offer, for comparison.