Halfords already have up to 50% off Child Car Seats, but as an internet special they're offering an extra 10% of child car seats until the 13th of December:

An extra 10% Off ALL Child Car Seats - Web Offer (Expires 13th December)
Halfords | Children's Car Seats | Baby Car Seats | Car Seats for Children

includes:
Halfords | Britax Prince Child Car Seat Alex
Halfords | Britax Adventure High Back Booster Seat Alex
Halfords | Maxi Cosi Priori XP Child Car Seat Black Reflection
Graco Logico L Child Car Seat Jet