More commonly used in the North, the British phrase "cracking the flags" is used to refer to hot weather. The flags in questions are not the flapping variety, but rather flagstones i.e. paving slabs or stones.
The implication is that the heat from the sun is hot enough to crack stone. Indeed, in some locations, "cracking the stones" is also heard.
Searching on-line, some sites imply that the phrase is also used to refer to really heavy rainfall, although that isn't something we've ever come across.
As an everday phrase, it's origins are not clear and obvious appearances in popular culture are few, although you would be likely to hear it from a BBC North West Tonight weather presenter.
It does appear in a different context in Shakespeare's Measure for Measure as "if you be remembered, cracking the stones of the foresaid prunes", but we're fairly sure the two are totally unrelated.
A quick trawl of the internet though and we turned up the following song by a London based singer / songwriter named James Henry:
Although the song might lead you to conclude that the phrase is also common in the South of England, note that his Facebook page says he originates from Liverpool.
Flagstones vs Pattern Imprinted Concrete
As driveway installers with over 30 years experience, we can tell you that the weather in Britain is very unlikely to be hot enough to crack flagstones.
Indian stone, block paving, bricks and other driveway and path materials are susceptible to our cold weather extremes though and cracking from ice is commonplace following a harsh winter.
For this reason, we recommend and install pattern imprinted concrete, as it is fibre enmeshed and up to 25% harder than standard concrete.
When laid properly with the required expansion joints, it will last for many years and you can be assured that it won't be cracking from the sun, rain or frost.
* Note, for those not from the North of England, that the t' is more implied, than pronounced.