Thought I’d give some feedback based on my experience.
I bought Colonia Express tickets via DirectFerries.co.uk and had no problems acquiring my bus/ferry ticket at Montevideo Tres Cruces terminal. Though, I do recommend bringing a printed ticket from DirectFerries
The bus to Colonia was surprisingly comfortable, the seats reclined and had a footrest which made me feel like I was in Premium Economy after the misfortune of having a transatlantic seat on a night flight that didn’t recline.
Immigration was very smooth on the Colonia side, and my exit stamp was quickly provided. The ferry was very busy, likely due to it being a Saturday in Summer. Immigration was *very* slow on the Argentinian side in Colonia. I ended up queuing for about 2 hours inside the Colonia Express terminal awaiting immigration.
It didn’t help that it was 40C when I arrived in Puerto Madero. In fact, the power kept cutting out every 15 minutes, which meant that migraciones had to wait for their computers to start up again - it happened to me midprocessing and I had to awkwardly wait, give my fingerprints and photo again. I was only asked to give the address where I was staying.
In any case, it ended up being 33 hours door-to-door from London to my apartment in Buenos Aires. It did save me a fair few hundred pounds but it was a gruelling, interminable, and exhausting journey. I would certainly say it’s not for the faint-hearted.