You really should be looking at a balanced diet coupled with exercise. As long as you eat good quality food and your calorie intake is in deficit then you will lose weight - simple cardio exercises during the day will increase this loss. 800 a day strikes me as way too little so your original plan to balance the food and Huel intake sounds better - maybe aim for an equal split of 800 of each.
In terms of fat loss the main aim would be to eat less and exercise more. Of course this is a very reductionist statement. The eating less would be better described as creating a calorie defeict, having balanced meals and eating foods low in saturated fats.
800 calories of Huel should be fine if the remainder is a balanced meal. However you need to consider that 1800 is still quite a low amount of calories. Without knowing your specifics (BIN, current activity etc) it is not easy to comment about this.
So far the best evidence for exercise specific to fat loss suggests regular High Intensity Training combined with fartlek (intervals).