stupot wrote:
What would be the fastest type of bike for a 7 mile(ew) commute.

The one Lance Armstrong is riding.

If you think that's a silly answer, the serious point is that speed
depends more on the rider than the bike.

Do I need a fast road bike?

I would say the main thing you need to consider is the terrain you are
riding on - since, I guess, that is going to be smooth tarmac, you want
a bike with slick, narrow(ish) tyres and no suspension.

As you are going to be using the bike for commuting, you also really
ought to consider mudguards, unless you like spending lots of time
cleaning your bike (or unless you are a fairweather-only cyclist, in
which case it doesn't matter so much).

For the same reason, you should also consider hub gears rather than
derailleurs, those this does limit your options somewhat, since most
bikes of the sort you are looking for come with derailleurs as
standard.

Whether you opt for racing bike-style drop handlebars or straights is a
matter of personal preference and comfort. And it depends partly what
you're used to.

The best thing to do is go to a bike shop and try a few - different
styles, different makes and models of the same style, different sizes,
etc, until you find something you are comfortable on.

Does ~£400 get a reasonable one?
And the silly question, can I spend less? :-)

You can get a brand new Dawes Comp Giro 200 for £300 - an eminently
sensible fast commuting bike.
url:http://www.evanscycles.com/product.jsp?style=5688

Or if you were prepared to go down the second-hand route, you could
probably get something much better for the same money.

But don't forget to budget for extra things you'll need such as lights
and a decent lock.

d.