Is it just a single journey you're after?
For flexible singles, splitting at Gloucester brings it down to £46.30.
Splitting again at Newport it's £46.00
If you're confident you can make a particular train, Advance fares are available Birmingham - Cardiff currently at £19.50 or £20.50, making it around £32 total. These Advances are the same price from Nuneaton which would save another £2 on the fare from Northampton (but take longer). Can't seem to get these cheaper by splitting (not many changeover points).
(Splitting at Hereford it's £37.50 (West Mids Day Ranger then Anytime Single), but that's no good as it would take too long for your trip)