How to measure thread sizes.
Measuring a thread size can be tricky, there are several ways depending on the thread type i.e. parrellel (straight thread) or taper (connical) , BSP (Bristish standard pipe) or NPT (National Pipe Tapered) etc. Then you either have a male thread (external thread) or female thread (external thread). We can go into more detail with regard the the number of threads per inch (TPI), thread depth, thread design etc, but for now we will keep it simple.
So, most UK and EU threads are BSP (G) threads , NPT is more commonly used the USA and Canada, so by accurately measuring the internal or external common UK BSP thread you should be able to determine what thread you actually have.
You can find more detailed technical information about different thred types here.
Helpful hint: Dont assume if you measure a thread as 25.4mm that it is 1" BSP, it does not work that way, see table below for guidence.
A male thread that measures 26.4mm is actually 3/4BSP..