planting under cloches

I don't think it matters when you go away or water, brassicas will most likely shrivel up under cloches, whatever else you do.

Try netting instead.

How big are the plants ? How many leaves and how big are the pots. In pots they will need to be watered .Larger the pot the longer a good soaking willl last,3" pot daily water to 10" pot every 3 days. Have you got a friend who will water for you.

I would plant them out and cover with Net / Fleece.There's not going to be any serious frost that would damage brassicas now.
If they are planted under a plastic cloche you will restrict the amount of natural water , but if they are well watered when planted you may be Ok. It depends a lot on what the weather is like , hot and sunny may cause the little plants to dry up before the roots have settled and found a water supply.

With a net or fleece cover they will get rainwater but have some protection from any cold winds or frost .Also protection from SkyRats(pigeons) and Rabbits.

The other question to ask is are the brassicas summer maturing or later. if they are summer cabbage/cauliflowers I would definately get them into their growing position asap. Others can wait  till you return from your Hols.

Would popping a few of those water retaining crystals in the soil around them help for just a week?

thanks for all the feedback

They are all between three and 4 inches high with three to four leaves on each plant.

I do not have enough netting to cover them all but do have some plastic polytunnels that are open ended. I could put some netting over the ends to keep the rabbits/pigeons out

ok - looks like i need some new netting then thanks