Yes, the Celeron range is old and single core and slower than your dual-core processor.
Yes, Vista basic doesn't have the Aero interface, but I suspect a £400 laptop would struggle with Aero anyway.
1GB of Ram will be enough - it'll have to be, I doubt you'd get more at that price point.
That's a very low budget for a laptop. Buy online rather than at somewhere like PC World; and have a look at the Dell Outlet.