So - Its me again.
Just wanted to update everyone who might still be interested. I have done slimming world for 2 weeks with my partner where I was using huel as breakfast (or lunch) for 80% of those days. For the record I counted Huel as my healthy extra B (which is essentially fibre, like bread - according to SW), and 2 syns. This seems to have worked fine, as I’m loosing at the ‘correct’ speed for my size and routine.
I have had a bad couple of days where I went WAYYYY over the syn count (it was pretty damn needed haha), but otherwise been very disciplined in making my shopping and treat choices. And despite having some bad days, I have fared very well!
Using Huel and slimming world I lost 1.2 kilos (2.8 pounds to the heathens) in those 2 weeks, with very little change to my routine. The only thing I changed was I decided to only get half my ‘treats’ on the online shop and the other half I had to walk to the store (about a mile round trip).
One thing to note is - I have always had some ‘tooting’ issues with huel, but using slimming world in conjunction, all the fruit and veg fibre and huel all at once REALLY shocked my system. My flat was a hazmat zone for the first week - my partner was not impressed 
To the Huel team - you can probably get in contact with SW to sort this out officially if you were so inclined (which you may not be!). I’ve only seen a couple threads like this so it might not be high on the agenda - but would be nice!