Cavity blockwork Vs Timber frame Garden Room (1)

[robinbanks]

Is there an accepted building option which comprises of a single skin block/brick construction + insulation?

Accepted by whom, and for what reasons?

My concern is moisture migration both in and out of the building

That doesn't really tell us what the problem is in your mind

I feel like i'm missing an option which is neither cavity or timber construction which i could consider.

Insulated concrete formwork? Single skin with waterproof render, spray foam insulation inner and bonded plasterboard?

You're mooting two-of-three construction materials arranged with one as the inner and one as the outer, two of which are potentially flammable unsuitable outers; mathematically you haven't got any options left that you haven't considered unless you add more layers or materials

Note, i'd be more comfortable with building out of block

Another restriction? Sounds like you're getting analysis paralysis (over a fancy shed, by the way) - take solace in the fact that most houses these days are timber frame or block cavity; they're the cheapest, most effective, proven build systems for this country, and they're accepted by banks nationwide. If there was a build system out there that was as effective for half the price (or whatever advantage), it would be popular..
(Try getting a mortgage on an ICF or slipform building here..)

We don't really put up residential pre formed tin coated insulation boxes like they do in Canada for example, but these "exotic" build systems are available (mainly industrial here) if you want to experiment

 

[Mike13]

Hi Mike,
Can you give me some more details on how that works please?

Unlock the Secrets of Decrement Delay and Enhance Your Building’s Thermal Performance with Dynamic…

Do you know that we spend about 80–90% of our time at home or in other public indoor environments? Are you aware of the great impact of…

blog.2050-materials.com

Decrement delay

Decrement delay - Designing Buildings - Share your construction industry knowledge. The term ‘decrement delay’ refers to the time it takes for heat to pass through an element of a building (such as an external wall or roof). Typically this is taken to be the delay in hours between the peak...

www.designingbuildings.co.uk

Preventing homes from overheating

Martin Twamley, UK technical manager at Steico, examines the factors behind overheating in housing and how the risks can be reduced

www.pbctoday.co.uk

https://www.concretecentre.com/TCC/media/TCCMediaLibrary/Products/Dynamic-thermal-properties-calculator-user-guide.pdf

that about covers it.
the idea is that you can then hopefully prevent the building from turning into a sauna at suppertime.