Almost every builders merchant will be able to supply something to meet MOT Type 1. Even BnQ and ebay sell it at a price.  It can be made of different materials but I believe the best is made from crushed limestone. Sometimes it's made from crushed recycled concrete or road scalpings. If it's from a reputable source and they say it's MOT Type 1 is should be ok. MOT is a bit easier to level and compact than a load of old bricks and rubble.

http://www.pavingexpert.com/subbase.htm#unbound

A proper job might have a layer of sand on top and then a DPM/plastic sheet before conc is poured. Even if you don't need a DPC DPM the plastic stops water running out of the concrete and helps maintain the right ratio while it sets.  

oops yes meant DPM.