It feels to me like having 144 profit on a 300 acre farm is closer to an accounting / tax choice then true measure of profitability. Another poster mentions writing off all equipment during the first year in that calculation.
It absolutely was an accounting trick, done in the name of TV ratings. This is Jeremy Clarkson - he's a TV personality and things done on his TV shows are not to be taken seriously.
The farm had a grain business, sheep, a farm store for veg and stuff, and grants from government for re-naturing part of the property. I have no idea how Clarkson calculated the 144 profit, but I suspect there was a lot of shenanigans do arrive at that number for effect.
Edit - I'll add that I enjoyed the show. While obviously done for shock value (similar vein as as Top Gear and Grand Tour), it did a reasonable job showing how complicated and expensive running a farm can be.