An update to earlier posts....
Discovered that the Society of Genealogists in London were more than happy to take any certificates I didn't want!
So the wife and I scanned all the certificates we had, that's both the ones which were relevant to our trees and the ones which were not, and then gave them to S.O.G. Just over 400 of them.
If I understood them correctly, S.O.G. have an upcoming project regarding BMD certificates. So at some future time it might be worth contacting them before purchasing from the GRO. In the meantime it looks as though they will happily take all those unwanted certificates - I'm sure they'll even take them from non-members!

Just in case the drives holding the scans die, we've attached copies of the scans to our trees (we use Family Tree Maker) and have then backed up our trees on line to Ancestry. So our trees and data and scans won't die with us.
Minor point: A typical scan of a BMD certificate as a .jpg file creates a file about 4 Megabytes in size. One scan on it's own is not a problem but a couple of hundred starts to create a huge file when all brought together, as in linking to a tree. To overcome this we compressed the scans to approx. 1/10th their original size. We used Microsoft Office Picture Manager but I'm sure that most image software packages could do the same. Compression doesn't destroy the image or data, and it makes a collection much more manageable.
Lubricated