What are the rules about combinability of fares, specifically what fares (or more likely fare classes) for the return can be combined with your outbound to form a RT?

Would it make sense to use the outbound and buy a new one way or RT for the return? Alternatively, can you cancel the existing RT and get some credit (or not) to use toward purchase of an entirely new RT ticket? If you're willing to pay 8000 GBP there should be some options.