Using the splitting tickets routine I must say I'm at a loss. The cheapest I can see is a combined total of ^64 which seems pretty expensive to me. I ahven't split the tickets up in 4 legs, would this make a substantial saving?
The difference between going to Rochester or Strood on the 14:25 from Cardiff is ^42!!!! (Strood from St Pancras, Rochester from London Bridge!)
Looking at
Avantix▸ Traveller, you might be able to beat the ^64 total without splitting or advance fares. Walk-up fares to Rochester and Strood are often the same when comparing like with like. From Cardiff to RTR or SOO there's an Off-peak Return (SVR) via Warminster/Salisbury for ^62. The restriction code is 8A, which is 'Valid on any train'.
The journey to London is slower via Salisbury, and will involve more changes, but as that route goes to Waterloo the journey will be possible without using the Underground, which will balance out the time and convenience, assuming there's no particular wish to use the high speed service from St Pancras. Two options spring to mind: go to Waterloo, cross to Waterloo East and join the Charing Cross to Gillingham service, which stops at Strood and Rochester; or change at Clapham Jn, go to Victoria, and take the main line service, which stops at Rochester.