I can't give a number as I don't know it - but I can give an opinion - which is fairly unbiased as although I'm a shotgun man, I have never had a handgun, or particularly wanted one.
ALL legally held private handguns were confiscated. That's because the authorities had a full detailed record of everything legally held. Illegally held handguns are a different question - those held by 'criminals' weren't on any record - and I very much doubt any were handed in. There is also a (small and decreasing) number of 'war souvenirs' from fathers and grandfathers forgotten in boxes in lofts and garages that are still occasionally handed in at 'amnesties' that occur periodically. I have never heard of these being used in crime directly, though I suppose that very rarely they may have fallen into criminal hands. The general view is that the illegally held guns come in from overseas. Border/security controls for sea and tunnel travel are not like airline security, and as most countries, there is a problem with drugs, alcohol and tobacco smuggling, the last two because we have relatively high duty rates on these items compared to some other European countries.
On crime, my impression that the 'crime' side such as armed robbery etc (banks, jewellers etc) - is largely unchanged because very very few of these used legally held guns. I guess there would be a very small reduction in 'domestic' crimes where a handgun was involved, but this was a very low level anyway, since it was never easy to hold a handgun legally and there was a vetting system in place.
I suspect that 'Googleing' may turn up some official figures, but those are my impressions as a UK citizen (and resident).
