That is not the case with national rail fares. You much use the outward before the return and a guard can demand to see the unused return portion when traveling on the outward portion of a return. To use the return portion first would be a breach of the National Conditions of carriage Rule 17 http://www.nationalrail.co.uk/static...tent/NRCOC.pdf and You would leave yourself open to prosecution, penalty fares etc.

BUT. HEx is HAL's toy trainset, is not a franchised national rail operator and it appears to have its own condition of carriage. https://www.heathrowexpress.com/File...ge-May2012.pdf I can't find an equivalent of rule 17 but you might want to check yourself.