leeks

Welcome to the site waliz, I have moved your topic to Grow Your Own.  Hopefully you will get plenty of advice.   

Could they have gone short of water during the hot spell? Leeks can often bolt if they dry out in hot conditions.

Totty

I would guess that Totty's explanation is the most likely - shortage of water will stress them in hot spells and trigger premature production of flower heads, although any sort of stress can do the same thing.

I had about half of mine bolt 2 years ago, and none last year (and none so far this year.) Same variety, same cultivation method - sometimes the life of a plant is just too complicated for we mere humans to fully understand! :

If you cut off the seeds heads as soon as you see them the stalk in the centre of the leek shouldn't be too tough, although it will be visible. I have no problem eating them as they are, although I suspect some people get their dissection kits out and surgically remove the offending items before cooking!