Registering account with Garmin with my first M2 but not with plan just yet-leaving in a month

No. The first month of a freedom plan starts as soon as you sign up. Even if you suspend the plan immediately, you will be in for that first month. Same with every subsequent time you activate a suspended freedom plan -- you get and are charged for a full month. This requires careful coordination to avoid paying for additional (sequential) months which you don't want.

All of that said, DO NOT wait until the very last minute to sign up for your plan. The forums are littered with sad stories from people who waited until the last minute and then were unable to sign up quickly enough to start their trip (servers were down, or there was a scheduled maintenance interval, or the device had a hardware problem, or they had billing issues, or whatever). 

If your first trip is 1 week, sign up 3 weeks or so in advance. 2 week trip, 2 weeks or so in advance. That is, wait until the first month will comfortably cover your trip. Don't cut it to fine, but don't wait too long.

Many thanks twolpert, if that were  the case I wouldn't have bothered for £15, but my first trip is 2 days short of a month so I'll have to buy two months worth of  messages.

If that's what I HAVE to do, I'll do it, but it's starting with a worrying precedent as these forums are "littered with sad stories" as you put it, but with stories of expensive mistakes because of a lack of knowledge.

The sad stories have a theme - "I waited until the day before the trip to sign up. Something went wrong and I couldn't get it done in time..."

semmyroundel said:

I'm not leaving for a month so don't want to start paying for a plan whilst I'm at home.

You need the device activated to do testing. Use several days playing around with the device, Garmin app and website until you know them as the back of your own hand. I am baffled when I meet inReach users out in the wild who barely know the basics of their emergency communications unit.