My thoughts exactly! Largs to Portsmouth, off peak return £136. Cheaper and more convenient (no changing stations at London) to drive, even travelling solo. For a couple, or a family, it becomes a no-brainer. Only problem being you have to either leave your car in Portsmouth or take the expensive option and take it on the ferry with you.
I have recently taken my bike to France (hence knowing the fare to Portsmouth) and taking it on trains over there was a doddle. Plenty of room, and trains actually leave and arrive at advertised time. Oh, and they don't stop halfway along the line and turf you onto a "replacement bus service" that won't take your bike

. No need for advance reservations in France either. Thoroughly enjoyable experience apart from the bit on this side of the channel.