The marris otter should be ideal for the pale malt. For the crystal 40, look for caramel 40 or maybe even 60. I think (might be wrong) that caramel and crystal are the same thing. Not sure about a black patent substitute. I'll have to look into it. Finally, instead if malto dextrine, you can use dextrine malt (usually known as carapils) instead. Add about 1/2 lb. in the last 30 minutes of the mash.
With either carapils in the mash or dextrine powder in the boil, you should have the same end result....added dextrines providing a little better body, or a little less thin. Hope this helps.