Plants during summer

when we were kids and used to leave for one month vacation (good old days...) my mom would put all the plants in the bathtub, fill it with some water (not too much though) and just go. The plants did not suffer and they were ok at our return.

We had a window and rolling shutters in our bathroom so they got some light too (but not too much).

It's an option, unless you find someone trustworthy to water your plants

Brico or some garden center also sell some devices that release water slowly. Basically you put a plastic water bottle upside down with this little "beak" attached to the top and the water is released drop by drop during a long period of time.

There are various solutions like automatic watering or 'programmateur d'arrosage automatique '
See
http://www.brico.be/wabs/fr/ami-renovateur/6281/conseil-du-mois/l-arrosa...

How are you travelling if in the car on a ferry or tunnel I doubt they will check

Here is a doc on the official rules. And, yes, you're limited to 5 plants.

"From listed European and Mediterranean countries, you are allowed a maximum of 5 plants unless accompanied by a phytosanitary certificate."

https://www.gov.uk/government/publications/bringing-fruit-vegetable-and-...

If your in a car stick them into a bag and then if they glance in they won't see them and you're assuming they even know the rules usually they want you through fast just take them.