For water jet cutting try http://www.controlwaterjet.co.ukwww.controlwaterjet.co.uk
They were very helpful and did a good job for us cutting out the parts for our hydraulic and pneumatic jaws, and chassis. (see photo on profile)
Just bear in mind that cutting through 20mm thick material you will get a slight taper/ angle to the cut as it goes through the metal. Give them a ring anyway, as mentioned I found them very helpful.
They can also supply the material and for us we found it was also a good price.

Guy