How many people are travelling? At least 2 because of the 'we' but could be more.
There is a little publicised special fare between Bath Spa and Gatwick Airport called the 'Gatwick Flyaway' @ £100. For the same price up to 4 adults can travel plus up to 4 children at an additional £1 each. It is also valid at any time, including peak.
The slight catch is that it is issued in one direction only, ie Bath Spa to Gatwick. However, it is a return so I can't see why you can't use the return part and throw away the outward, if that suits your plans.
It is only available to buy from GWR, valid on the route via Guildford and Reading.
Also if there are more than 2, you could alternatively use Groupsave, where each would get a third off regular tickets (but not the special offer above).