Evasion. Or rather attempted evasion until the journey is complete, when it becomes straight evasion.
Next.
I did exactly this a couple of years ago on a journey from Highbridge to a suburban station in Bristol. Neither station had a ticket machine or staff and Great Western had obligingly provided two units for which there was no gangway connection. Guess where I sat? But I have sat in the front unit since my trainspotting days, it's my participation in these forums which led me to expect that I would not have to pay as I now know the train has to be given the rightaway from the rear unit, so you may argue I OUGHT to have deliberately sat in the rear unit to be honest.
If the guard had somehow managed to get into the first unit I would of course have had the means to pay and done so, but I literally had no opportunity to pay. On the return journey I of course still had no ticket and there was only one unit, but the guard appeared to be too busy to sell me a ticket anyway.
If I had been intending going to Temple Meads I could have alighted at Bedminster and caught a bus for the last couple of miles rather than pay on arrival at Temple Meads, would that have been a worse evasion? I wonder how may people alight at unstaffed stations close to a city centre if they have not had to pay for the journey so far!
My take is simply that the rail industry is fundamentally fraudulent and dishonest in its charging structure and the way it treats passengers so whatever you can do to regain the advantage without committing a criminal offence or telling lies is fair game - whether split ticketing, loophole tickets or strategic sitting!