You need to get rid of the slugs and snails. You can get organic slug pellets that contain ferric phosphate so they're not harmful to other wildlife. These will help, but there's no guarantee that the slugs and snails will find and eat these before they feast on your dahlias.

The best and quickest option is for you to go out after dark with a torch in your hand and some boots on your feet, pick the slugs and snails off your plants and stomp on them.

Moving the plants into containers won't really help - the dratted critters can climb. If you eliminate the slugs and snails for several nights, your plants should recover.

Or you can now buy nematodes which destroy slugs and snails - available online from Green Gardener - organic, doesn't matter if birds and hedgehogs eat the dead snails and slugs because the nematodes aren't toxic. It's called Nemaslug, needs mixing with water and watering on the ground.

Copper rings, they apparently don't like crossing them, I used them one year for Hostas in pots. You can buy the copper on a sticky tape to put around pots.
Maybe next year get the Dahlias to a good size in pots then slot the pots into the border and the plants will have got past the new growth and sappy stage and be able to resist the beasties better!!
I too am growing Dahlias this year and have them in pots on the patio, so far so good.........hmmm.

thank you for that advice im going to buy some copper on the tape and give that a go hopefully it will work fingers crossed then !

or i might put them i pots and put them on my gravel by the harbour until their bigger .someone told me they dont like anything coarse to move on even roofing felt works apparentley!

I put a ring of eggshells around my emerging Dahlias

I've given incorrect info above - the nematode treatment works for slugs, and only reduces snails. But Green Gardener also sells copper tape, copper mats, etc., for reducing snails.