Analysis of UK / RSGB licence database shows that approximately a quarter of the 83,000 amateur / ham radio licences in the Ofcom database have been revalidated
Ofcom introduced a "Lifetime" licence requiring 5 yearly revalidation in December 2006 but it seems they didn't set up an automatic mechanism to revoke licences that were not revalidated. All licences issued during the past 8.5 years remain in the database. It is presumed that at some point over the Summer Ofcom will at last revoke the unvalidated licences.
The 61,000 validated UK amateur / ham radio licences is not the total of UK amateurs since a large number of amateur radio operators hold multiple licenses. Ofcom have been allowing amateurs to revalidate two Full licences (old Class A, Class B, Class A/B calls) as well also allowing those who have done the RCF exams to revalidate up to three callsigns (Fnd, Int, Full).
Essex Ham Validation Guide - http://www.essexham.co.uk/news/remember-to-validate-your-licence.html
Ofcom's latest amateur radio licence statistics - http://licensing.ofcom.org.uk/radiocommunication-licences/amateur-radio/guidance-for-licensees/monthly-stats/
More than one licence. Must I revalidate all of them?
Ofcoms frequently asked questions section states the following
You should only have one personal licence. You should not, for example, have two Full licences that reflect the old 'A' and 'B' licences respectively. However, we know that some amateurs have more than one personal licence and, for the purposes of revalidating, you should revalidate all or any licence(s) that you hold.
If you have more than one personal licence, it is likely that we shall be contacting you in the future to ask you which one you wish to keep and we may require you to relinquish all but one of your licences.
Read the Ofcom FAQs - http://licensing.ofcom.org.uk/radiocommunication-licences/amateur-radio/faq/?a=0