If you are in UK you are in Heaven !!
Ask them straight.
Worst case, they'll ask you to order through a shop .
I would go to them only for certain very specific parts, such as a Power Transformer and such; generic electronic parts or pots/switckes/jacks, etc.can be found in good electronics suppliers.
Now, incredibly, some parts Marshall will not sell you, such as Tolex, speaker cloth, front panels, even Marshall plastic signs and logos, etc. , can be easily ordered from some specialized USA suppliers.
What do you want to do?