Thanks for re-opening.

TPTB=An abbreviation for the "the powers that be" avoids tedious discussion as to exactly WHICH official or government department is being referred to.

As regards martial law in the UK, I am not aware of it ever being been declared in recent times.
Likewise in the USA.

I consider it unlikely that martial law will be declared in either the USA or the UK.
It COULD happen though, and without veering into political argument, it seems reasonable to state that government powers are likely to be very considerably in excess of those powers whilst times are normal.

Requisitioning of goods and supplies, restrictions on travel, and the directing of labour are foreseeable consequences.

Disobeying instructions whilst under martial law could be a very high risk strategy indeed.

There is IMHO something to be said for a well buried cache of supplies for use after any serious or major disaster, including but not limited to martial law.
Floods, fires, tornadoes, heatwaves, blizzards, earthquakes, and other disasters are more likely and more worth considering.