The Ultima and Etched Pixels products are produced to two different scales. The first of these is traditional UK N scale (1:148), the second is the traditional fine scale modelling scale of 2mm/ft (1:152). The differences between these two scales are small enough that it rarely matters.
The one thing to be more cautious about is mixing two coaches of the same type in differing scales. At that point the length difference in particular can be more obvious. Nevertheless it is not a big difference in most cases, and many modellers happily mix the kit built Stanier 68' and 69' coaches with 57' ready-to-run products.
We are working on making the product pages indicate the scale of the kits. For many of the small parts the scale is not indicated. You would need a micrometer to tell the difference, and in many cases whether it is 1:148 or 1:152 might depend upon the actual casting shrinkage of any given component!