Mapei Ultraplan - around £18.00 per bag. Flexible - a couple of diff dypes - levels from 10mm to around 30mm in one application. If you ix properly & add 5.5 litres of water to a full bag, then you only need to spread around roughly with trowel. The compound will level itself perfectly. The advantage of this is that you use less adhesive and tiling is much quicker as you have not got to worry about adjusting your bed depth because of an unever floor.
Another type that is supposed to be better is Mira X Plan - this can be delivered nationwide - try phoning Specialist Tiling Products In Harrogate, they will be able to help. Similar price.
Beware of cheap self levelling compounds.