The only current British Standards referrencing 'pixel' that I am aware of is BS ISO/IEC 9636-6:1991:
Note: the 'BS' is for British Standards; the ISO/IEC means that the BS is identical to the 'ISO' (International Standards Associaton), 'IEC' (International Electrotechnical Commission) standards of the same number; the 9636-6 means standards 9636-1 through 9636-6 inclusive. The standards are available for a significant payment for those who like reading such things.
The simple answer is 'a pixel is a pixel and has been since 1991' on the world's commercial hardware.
What the client may be referring to is a British Standards 'Kitemark': a logo using the letters 'B' and 'S' looking something like a heart. To North Americans it is something similar to the 'UL', 'ULC', and 'CSA' labels for manufacturers plus the 'ISO' quality programs aimed at various levels of commerce.
This obviously has nothing to do with 'pixels'.
Perhaps your client is confusing two different programs/standards?