From 7,200,000 to 10,300,000 at about 2007 is the number of domestic cats in the UK. The point is we don't know an exact figure because no one is counting! These are estimates and as you can see the figures relate to four years ago (today's date: 23rd Nov. 2011). There is a general slow climb in the numbers of cats and dogs as the population increases in the UK. Therefore the figures will be slightly higher for 2011.
There are an almost equal number of dogs in the UK. The general view is that there is a similar number of feral cats as domestic cats I don't believe that is the case in the UK. In the USA it is probably true that there are similar numbers of feral and domestic cats (about 80m each).
The figure of 7.2 million comes from a research document entitled: Number and ownership profiles of cats and dogs in the UK by J. K. Murray et al from Department of Clinical Veterinary Science, University of Bristol, Langford House, Langford, Bristol BS40 5D.
The 10.3 million figure comes from Pet Food Manufacturers Association - PFMA. You can see a page on Ownership of Cats in the UK.