I agree there is a terminology issue here but I am not sure that the offences are different. The facts of the offence are the same in the sense that a man performs an act of penetration on a woman without her consent or without his reasonable belief in her consent (I don't know the exact statutory wording) - the difference arises in the likely sentence. You could argue that there is a factual difference in the sense that in one case the parties are married and in the other they are strangers, but the difficulty is, there are always going to be factual differences beyond the actual offence to give "degrees", but that is where the sentencing provisions step in.
And yes, there is a definition issue here in Sharia law because their definition of rape is entirely different to ours. On the Sharia definition, presumably a single woman attacked and penetrated by a single man cannot have been "raped". Unless of course their definition of adultery is different (which I rather suspect it is).