the site only says
Why doesn't the site show me the cheapest fares?
You may not have been shown the cheapest fares for a number of reasons:
1. The cheapest fares may be offered as a promotion. Promotional fares may only be available from selected outlets or agents and we may not be able to sell these.
2. The site searches for the five fastest trains in each direction, and any slower direct trains. The times, dates or routes you have specified may not have any very cheap fares available. You could select other dates, times and routes to see if cheaper fares are available.
so I don't think they actually have an option to switch from fastest to cheapest. Now I know what trains i want tho i will prolly get it from the station. I have a £5 voucher i want to use anyway.