I think (given that 2.2 million lab mice were used in England in 2007) that computer mice probably just about outnumber lab mice. There are about 25 million households in Britain, and I don't expect that each of them contains forty mice, so computer mice made by Logitech alone may well outnumber house-mice in Britain.
I'm always impressed how common humans are as an animal their size; the Great Wildebeeste Migration in Africa is of 1.5 million wildebeeste at 150 kilograms each, which is fewer individuals but probably a bit more mass than the daily Commuter Migration into and out of London.
Thanks to a very useful dataset pointed out by shimgray, I can say that there are more people than mice in the UK; however, field voles outnumber humans. For every other person in the UK there is a mole.