BMI vs Centile - 4 year old?

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I recently had my 4 year old weighed (fussy eater, very active and slim!)

The HV weighed him and said he is on the 25th centile,
however when I put his weight and height in to the NHS BMI calculator it states he is on the 6th centile, so whilst still healthy, it is on the lower side.

Which one do I go by? He was born on the 6th centile.

I'd go by the WHO growth charts. BMI are a bit of a weird way to measure a child, but that's just my opinion

Growth charts in his red book.

Our paediatrician completely ignores BMI. My daughter is on the 1st percentile for height and 25th for weight which by BMI puts her on the verge of overweight. He is very pleased with her weight and comments on how good it is given her dietary limitations for allergies, reflux and toddler diarrhoea.
She is sturdy looking but her paediatrician and dietician both said that she has a normal looking build when I have queried the difference between her height and weight. They count a weight and height that are between 2 full percentiles of one another as a normal size. Her height is always between the 1st and the 2nd percentile so on the limit of this.
She outgrows clothes based on her height and still wears some 1-1.5 aged clothes at 3.5 although mostly in 1.5-2.