The First Class prices seem to be out of the realm of "pricey" and well into the region of "mindblowingly stupid". Who pays £233.50 to get to London and back (about a hundred miles) on a Friday night? On a train? Is there actually anyone, or are these fares some strange artefact of the booking/pricing system?
Surely for that money, you'd hire a limousine instead, or just buy a car, and therefore either be free of parking problems or be able to change your return plans easily.
I might suggest this as a Top Gear challenge, actually. How ostentatiously can you get from Bristol to London and back on a budget of £233.50, given a week to make your plans?