In no way do I mean to criticise anyone or put anyone down, but this is a classic example of the complicated fares system we have in the UK. Good luck to the OP in getting a good deal...Grand Central seems the best option and saves any hassle getting to Leeds. Also good luck to your Mrs on the show!
But looking at it another way, a £67 return (each) option with Grand Central is not a bad deal nor is complicated. Obviously the complication arises when we have to delve into splitting tickets, not everyone will be aware these options exist but it does dispel the myth (for me anyway ) that all tickets are overly expensive.
Naturally those used to split ticketing and taking advanced fares will see Bradford to London for that price as expensive, however, the casual user would see £134 as very reasonable considering the OP (not a regular traveller) was quoting £120 in fuel, without wear and tear on the vehicle and the hassle of driving the journey, not to mention potential parking costs at the capital.
As it stands though, you are right, the OP has initially looked at these very fares and become unstuck with not knowing which option to select (not his fault) so was quoted much more for what he believed to be the same ticket. Many would just take this £186 fare for two as the norm.