Indeed.
You’re logic is broken.
It’s called induction hypothesis… one step is true, then follows for others.
The illustration of two people agreed to a price of one token is allusion to a market of many agreeing the same. There is no difference in the reality that the market means nothing, until the token is recognized - which is at the boundary with a legal object - be that real world object or fiat.
Some would argue fiat is bogus but that does not appreciate what fiat is - still, there is a confusion that fiat causes in this… making people jump from what they know, to something new and making assumptions about it. Point being, a reasonable person would not necessary anticipate that the HMRC or any other would fall down that path they have chosen - it is not consistence with other CGT or with a good proper sense for what cryptocurrency was and still is … there is a tipping point perhaps where a token is stable but I don’t see any that could not collapse to zero in a heart beat.
Some would wonder there is an assumption there that people are subscribed. Personally I don’t mind that Government does look to raise taxes in that way but they have got to do it right.
and here is the problem:
There is no way to know… why suspect it?.. that is not reasonable from them.
As above, the market of two is no different to the market of 200 or 200,000 delusional people - who know not but suspect the value that they could realize… the test, is only at the point of action into fiat… on shallow markets the spot price does not reflect the reality of what can be realized… that should be obvious.
The onus is on the HMRC to set out guidance that is not ambiguous… that does not risk a big unknown; that does not post date action against vague guidance that is fishing for what they can exploit in the future… far beyond the reality of what was participation is… they are by taxing transaction stifling innovation… and risking new markets crashing by forcing payment of some [insert random decision making] about how much is owed by whoever is gullible enough to engage with ambiguous. Some will follow through the process they set out, expecting it to be ok and getting snapped in this trap. Tax is meant to be simple and boring… what HMRC is doing is complex and stressful.
I could go on about other errors but the process is broken… they have not a good feedback loop to get this corrected. As such, default has to be to treat with contempt that broken process - to do what is possible for an individual but no more.
Again, this is not rocket science but they have fudged up and it needs a fix… which is as was suggested is tax at the fiat boundary and shoring up their arguement for future. If they really must attack transactions, then they should be looking to limit that damage to markets that can sustain that burden without compromising development of the innovation - atm ALL the markets are about innovation seeking potential utility, not actual.
Given this is relatively obvious… I suspect they are deliberately trying to stifle crypto; so, they can inflict a centralized and hobbled controlled version of it. We know that BoE and others are researching this and there’s reason they might - I replied to the open call on Digital Currencies a while back and suggest why… but to kill innovation is stupid… not that that will stop them! 
Enough on a boring topic for a Friday evening… HMRC wasting my time!!