Bertie, I am on my pre op diet,this is what I am allowed each day. a slim fast for breakfast and lunch. 200g Very Lean meat and 6 tablespoons of cooked veg or one healthy ready meal (less tha 450 cals) plus one portion of fruit. or 4 slim fast.
Also at least 2 litres of liquid including bovril, oxo, tea, coffee, NAS squash, diet sodas. NO alcohol. You may use any herbs and spices in your meal. You may add sweetener to your tea or coffee.
I think you should ring your team to find out if this is ok for you.
HTH