That would be an excellent question to ask the person that gave you this advice. The only thing I've heard is that eating a low carb diet may cause a false positive result on the OGTT.
So, for the standard lucozade providing 70kcal per 100ml the breakdown is as follows
21.8g sugar per 250 ml.
Their
website says something different: "Nutritional Information (per 100ml): Energy: 297kj / 70kcal, Protein: Trace, Carbohydrate: 17.2, Fat: Nil.", which is consistent with the calories you quoted (1g carbs = 4 kcal, so given that it's sugar water you'd expect about 20g carbs per 100ml)
Using those numbers, you'd get 440ml of Lucozade.
Caution: I just read that the NHS, apparently, uses Lucozade in OGTT. I think that's a mistake - it's called
Glucose Tolerance Test for a reason and the WHO specfically calls for (page 48 of
this report) 75g of glucose or partial hydrolysates of starch to be used. In contrast, table sugar and high fructose corn syrup/glucose fructose syrup is only about 50% BG active glucose with the rest being (differently metabolised) fructose - that's why there used to be quite a lot of "diabetic sweets" made with fructose that wouldn't affect BG much .
In fairness though, Lucozade is rather unforthcoming with that fact - it says "Drink. Think. Do. Glucose" (
product page) but you'll need to read the ingredients to see that it's actually made from high fructose corn syrup.
EDIT to add: There seems to have been a recent change in Lucozade - both Wikipedia and an unsourced alleged GSK document suggest that it used to be made from real glucose syrup ("The sole source of carbohydrate (CHO) in Lucozade Sparkling Glucose Drink is glucose syrup (liquid glucose)" in a 2003 GSK document)