Rachel is a very common given name. “Rach” is a fairly common nickname for it. “Rach” is not a common given name. (This matches what I said above.)

I just took a look at some baby name sites to try and find some statistics. I actually can’t find a single person named “Rach” because all the sites assume I want statistics for the long form, even when I’m on the page for “Rach” and they also have a page for “Rachel.” I’m interpreting this as being given the short form as your name is extremely rare.

Given that OP says this is a common English name (it’s not), I have to imagine that they’re referring to the common short form of Rachel. Pronounced as just the first syllable.