I buy mine from a local factory that make these boxes. I just ring and order then go and pick them up. However, I am in Scotland, not Liverpool anymore, but there are bound to be factories that make them around Livepool if you check your yellow pages.
I find the poly boxes the best thing I have used to send parcels, because they are so lightweight, but so strong. (Cardboard is heavy and easily damaged) Prices start for the small boxes at about 30p each. For a large box which is known at the Factory as a "coffin" it's £2.75.
I buy my bubble wrap on Ebay as you can get a really huge roll delivered free at a fraction of the cost in the shops. I also buy "Fragile" tape on Ebay.