Just been to Halfords.
Not sure when this started, or when it finishes, but I've been waiting for this deal to return (I think it's better than the restrictive Turtle Wax 3 for 2 etc).
A particular favourite of mine is 3 x 5L Turtle Wax Extreme nanotech wash and wax, working out at £2 per litre. An excellent and underrated car shampoo, in my opinion. Much better than the slightly cheaper TW Zip wax (it's reduced in store), and as good as some more exclusive brands, at a fraction of the cost. That's the next five years sorted!
3x Autoglym High Definition Wax works out at £30ea (but still not as good as flea-bay @ £24.98ea).
For ease of use, the excellent Autoglym Aqua Car Wax 500ml works out at £9.33ea (better than I can find anywhere).
Feel free to add your favourites. Cheers.