Hey mate, deffinitly dont buy a bike just yet.. wait until your validated, with £450 to spend you could easily get a top spec 20" bike. Don't know if you know this, but 20" is also called a 'mod'. Just keep doing posts like that, good spelling, good questions/useful answers and you will be validated very quick. Just dont ask to be validated.
Good luck, deffinitly get a bike of here if you have that money.
Hope i helped
Kieran