If you go into Nominet Business Intelligence, then choose Registrars->Registrations and pick "Data Table" you can see some data. There's no total, but you can see the number of domains registered each month.
For the last 12 months (i.e. 2014):
Jan 156,726
Feb 147,952
Mar 152,361
Apr 138,125
May 134,096
Jun 223,896
Jul 155,440
Aug 139,649
Sep 140,304
Oct 155,162
Nov 139,643
Dec 109,575
For the whole of 2014, 1,705,929 domains were registered, a decline of 9.34% on the previous year (I know that total is different from what you get if you total up the 12 individual months above, I am only passing on Nominet's own figures)