You can't excess away the origin of a return ticket (which is what you would be doing for the Peterborough to London ticket) and it can't be a change of route excess because you are changing the origin/destination of both tickets not the route (e.g. any permitted).
There is perhaps a very slim chance that a ticket office might issue an over-distance excess for the Peterborough ticket to make it into an Oakham ticket, and then issue a further change of route excess to all travel via Leicester. However, there is no clear entitlement to an over-distance excess, especially after the outward journey has been completed, so if there is a chance of wanting an alternative route buy the through ticket at the outset and accept that the 50p saving has drawbacks as well as a very small advantage.
