(2-part post):
Where on earth are you searching for ticket prices? While the 'Anytime' return rail fare from Evesham to London is £61.00 (which does, indeed, result in a total fare of over £120), the 'Offpeak' fare (which applies to all services, both outbound and return, between Evesham and London at weekends) is only £33.00 (giving a total, for two people, of £66.00).
With the possible exception of Megatrain services (which don't operate on your route) there are only two websites which should ever use when seeking the cheapest rail fares. (They'll both give the same results). One is the website of whichever train operator serves the relevant routes. The other is that of National Rail:
http://www.nationalrail.co.uk/
All available fares, other than possibly for some Megatrain services, are always listed on the National Rail site. Other sites, such as TheTrainline.com, have no access to any cheaper fares and often add on a booking fee. (Further, as a former rail station supervisor I know only too well that the independent ticketing sites have a nasty habit of selling tickets which aren't valid for travel on any trains at all!) There are no 'advance purchase' cheap tickets on that rail route (on the dates which you've specified).