Friday evening is by far the busiest time of all on Britain's railways, so there simply aren't any bargain fares on offer on most routes at that time.
The total return fare for 2 adults and 1 child (as the baby travels free) is £139.05 or £106.05 if one of the adults holds a Family & Friends Railcard. (A Family & Friends Railcard costs £30 for one year and can be purchased at a staffed station or online. A 3-year version costs £70 but can only be purchased online. It gets up to 4 adults one third off their fares and up to 4 children 60% off their fares for a full year, or for three years, as appropriate).
None of the fare-splitting websites seem able to offer anything cheaper. (Sorry!). However a bit of manual experimentation with the National Rail website shows that you can actually cut the total fares by booking from Newbury to Castle Cary and then from Castle Cary to Exeter St David's:
Doing it that way (without a Friends & Family Railcard) will cost you £55.50 return for the Newbury-Castle Cary sector and £53.85 return for the other sector, making £109.35 in total.
http://www.nationalrail.co.uk/