Insulating your loft is important as you could be losing up to 1/3 of your heating cost through your roof.
You may be surprised to know that the current recommended depth of insulation is 270mm (10.5 inches). This is quite a thick layer!!! You may find you already have some insulation in the loft. If it hasn't been done for many years it will probably be quite thin.
NB some councils will give grants for loft insulation. Check it out you may be able to save some of the cost!
There are various types of insulation available. For a DIY er the choice is between a:
Mineral wool Quilt eg Rockwool sold in rolls in big bags of varying width and thickness.
Loose fill . Sold in bags of granules and poured between the joists.
Given that the current recommended thickness is 270mm or 10.5 inches I would recommend using Mineral wool. You should find that this is readily available at your local builders merchants or big DIY stores.