BT Update, the booklet sent out with BT bills contains the following statement about the new prefix. It is also on the website.
Quote:Additional number range for London
From June 2006, as well as London numbers beginning 020 7 and 020 8 you may also see 020 3 numbers. This new number range is being introduced because London is running out of numbers. 020 3 will be used for both inner and outer London. If you'd like more information visit the Office of Communications, Ofcom.
Doesn't really make it clear that it's not a new code, but a new range of London numbers beginning 3. Also, I don't understand why it says June 2006.