It’s a fair bet (but I might be wrong) that they were using something like the cambridge diet plan.
I actually considered using this but decided to use Huel instead for one very simple reason - £££
Cambridge diet plan is around £50+ a week for enough shakes to be on an 800kcal diet, whereas using Huel I can do the same thing myself for around £20 a week. I don’t actually go quite as low as 800kcal, but I cycle 8 miles to work and back some days, so I wasn’t sure that was the best idea anyway - it’s possible the cambridge diet plan could have accommodated that, but I’m fairly confident it would have still been much more expensive than Huel.
The big upside of the CDP is the 1:1 support. Personally I’ve found that religiously logging absolutely everything I eat on MyFitnessPal, as well as being accountable to my wife, works pretty well in leiu of that and I’ve managed to resist cheating pretty well so far.
The added benefit of using MyFitnessPal for me, is you can also use their other app, MapMyRide, to log exercise just by doing it and letting the app track the GPS on your phone, and it syncs your calories burned back into MyFitnessPal. Both apps are free but there are paid “pro” versions with more features - personally I just use the free ones.
Personally I don’t have a huge amount of self discipline so I’ve been kind of surprised that this has been working well so far, my intention was always to try Huel out and keep the CDP as a backup option that I’d go back to if the “DIY” method didn’t work.