It sounds like your just after a simple base, a diorama is somthing different.
Great North Roads has some very nice stuff and a couple of there bases may suit your needs. I have used them for quite a few things and the stuff is very nice to work with.
https://www.greatnorthroads.co.uk/
But i would really think about making your own, as it will be cheaper and your can model it just as you want it. I use Celluclay to make terrain but other people use plaster or other products.
I am a Norfolk man and i glory in being so
On the bench: Airfix 1/72nd Harrier GR.3/Fujimi 1/72nd Ju 87D-3