Michael Faraday.
Sir Isaac Newton.
James Clerk Maxwell.
(among many others)
There are quite a few folks on there that have undoubtedly placed themselves among the very highest class of contributors to the advancement of humanity.
It isn't, and shouldn't, be a matter of just picking your top four people and saying they should be on the notes forever. It changes with each new note series. Newton was on the D-series £1 note. Michael Faraday was on the E-series £20 note (only withdrawn a decade ago) (source: http://en.wikipedia.org/wiki/Bank_of_England_note_issues#Den... ). Why cycle them back in so soon? Give some others a chance.
You're right about Maxwell. The other two have already appeared on banknotes. (The ones highlighted in grey have already appeared on banknotes, though there's some inconsistency between the highlighting and Wikipedia's list of people on UK currency.)
Other scientists, mathematicians, and technical innovators already honored: Kelvin, Darwin, James Watt, Florence Nightingale, George Stephenson, Alexander Graham Bell, and Alexander Fleming.
From the list[1], authors such as Kipling, Potter or Wilde would be crowd pleasers, but a figure like Winston Churchill would likely win if this were decided by popular vote.
Saying that, recent Bank of England notes[2] seem to sway much more heavily towards engineers/scientists (Newton, Wren, Stevenson, Faraday, Darwin, Boulton/Watt) and social/economic reformers (Nightingale, Houblon, Fry, Smith) than artists (Elgar, Shakespeare, Dickens) or military leaders (Wellington)
The only player out of that list who compares is Scholes; Paul Scholes is the best midfield player of the last 20 years according to Xavi.
Neither Rooney or Gerrard are fit to lace Beckham's boots. Rooney has been a shortlived wonderboy, and Gerrard admittedly has been fantastic for Liverpool, but has had one good game for England in the 5-1 victory over Germany.
Beckham's vanity as a person shouldn't impact the reality that he was an extremely good footballer.
Well, the point is, he's a distinguished, popular, and famous Briton because of his skills in sport. It's not outside the realm of imagination that he might deserve to be on a bill, though that honour is usually mostly reserved for politicians or royalty.
I see a few names on that list that shouldn't even be in the queue in my opinion.
David Beckham, Jonny Wilkinson, Robbie Williams, Michael Vaughan, Terry Wogan, Princess Diana...