What was your EXACT ticket routing, point and date of sale? Even better would be if you have the fare code. We can then probably help you further.

Recalculated at historical fares means nothing other than at the same fare you paid at the time you purchased the ticket, subject of course to change fees and possibly a differential for a changed routing, cabin class or booking code.

You may not find it too expensive to change to the same routing just 1 week later.

EDIT: Assuming point of sale Dublin. LAS to LGW with onward BA LHR-DUB is available for Sunday 10th September. You'll probably find this a simple change-fee transaction.