This is likely to depend on the rules of the fare you booked. If the fare allows stopovers, then (for the change fee and subject to availability) you can insert a stopover in London - and whether or not you then fly the final leg after the stopover is a matter for you.

If the fare doesn't allow stopovers, then simply cancelling the final leg will trigger a re-price of the ticket, almost certainly to a much more expensive fare.

The costs of either of these options may well exceed the cost of shipping or posting your checked luggage to the UK, allowing you to fly the return sectors with hand baggage only - and no-show the final leg.