Using Return Flight for One Way Only?

Here's the deal, I'm looking to fly to NY next year and return on the QM2 to Southampton.
However, it appears that return tickets are cheaper than one-way ticket....Now I'm sure both VA and BA have this covered in their small print, but what would happen if I only used the outward bound part of my ticket?
Many thanks in advance,
However, it appears that return tickets are cheaper than one-way ticket....Now I'm sure both VA and BA have this covered in their small print, but what would happen if I only used the outward bound part of my ticket?
Many thanks in advance,