Hello Gareth,
The National Rail journey planner is full of glitches like the one you have found. Not only are some of the routes and times silly but some of the fares that it comes up with are guaranteed to make your hair curl
Talking of fares. I would have thought that it would be very difficult to get a cheap ticket at that hour of the day on any of the commuter services going into London. We have quite a few younger lads on the forum who do a fair bit of travelling so hopefully they will be able to offer you some advice when they come on-line.