Cannondale Topstone Tiagra Upgrades - Musts, Must Nots, and MustArd It's been a while since I last rode a bike but having bought a Cannondale Topstone Tiagra only a few days ago and finding the hidden streets and valleys of a town in which I've lived for many years I've reignited my passion. 

Outside of riding my head has been frazzled by research into how to improve my bike and essentially gear up towards bikepacking with a journey from North to the South of England (Carlisle to Bristol 380 miles)

Certain articles recommend switching the Crankset of the Tiagra. I've read about the Selle Anatomica seatpost as a seat to save your perennial, Dropbars, Tape and everything in between. 

Any recommendations? Musts, Must Nots and MustArd.
We can't tell you what upgrades you should make, because we are not you. We don't know what your preferences or priorities are, what kind of rider you are or what riding you want to do (apart from knowing you have a long distance goal in mind).

Go ride the bike. Make a training plan for your long distance ride. Go ride the bike more. Figure out what works and what does not. Decide where your money is best spent. Consider that money may probably better spent on gear and accessories (clothing, shoes, computer, tools etc) than on bike upgrades.

Addressing some things you mentioned:

Replacing the FSA crank with a Shimano one. Yes you could do this, but why? Is the FSA crank actually deficient? Crank sets are relatively expensive so see my point about where to spend money above.

If long distance riding is your goal then switching out the saddle is a definite possibility. Stick with the stock saddle and see if you have problems with it first though. Saddle choice is unfortunately down to experimentation with different brands and models. You also want to make sure you saddle is set up right for you, some people like a little more forward tilt than others.

If you are thinking about a suspension seat post, good saddle fit and adjustment is far more important for 'saving your perineum'.