This is one way I would consider spending that £400.
Head Unit
Alpine 9874RR - £114.99
Front Components
Infinity Ref6520i - £69.99
Rear Coaxials
Alpine SPE-17C2 - £24.99
Sub/Amp/Box/Wiring
Infinity Ref1600a + Infinity Kappa 102.7 DVC - £189.99
TBH though I would buy from ebay/second hand, bargains to be had especially on amps (which tend to eat most of the budget). My Infinity 7521a was retailling at £120, bought for £58 from CAS; Alpine amp was retailling >£250, bought for £46.