Hi Bushyboy. Not sure how much previous experience you have of stamp collecting, but Machin collections can get into very advanced territory, such as identifying varieties based on the appearance of the Queen's hair curls, gum types, even the thickness of bits of the numerals and so on.
If you're starting out, you may find it unfathomable. But don't be put off - a basic Machin collection is great fun, and you may want to start your own. This site is useful: https://adminware.ca/machin.htm
of course, if you know about stamps and collecting generally, ignore this post!