Google suggests a Syn is 20kCal so 50g of Huel wil be 200kCal equivalent to 10 Syns, though why the dolts had to make up their own calorific unit, god knows.
If this is important to you you should probably double check this as I know nothing about Slimming World. Another google hit argues a Syn is (fat+carbs)/5 (where we’re converting from mass in grams), making 50g Huel roughly 5 Syns.
There are however deviations. It all seems to be part of what SW would like you to believe is some special secret measuring system they have which will lead to weight loss when you follow their guidance. It smells rather wrong to me particularly basing their business model on this secret measurement. I prefer the open peer reviewed process of science to some contrived pseudo-scientific bs.
That’s only my opinion though. If it works for some then good for them. I imagine some of the individuals running groups do offer genuine support & help. I just don’t trust the company that’s selling this franchise to them. Sláinte.