any chance of a picture please ? I don't suppose you've got a mink or an otter that is going in and out and making it look like the plants are being pulled out ? id put some sand all the way around the edge so you can see the footprints . just because you don't have fish doesn't mean the wildlife knows that .

Highly unlikely to be cat or birds, unless you have a heron in the area... as NP says a pic would help

There are no mink or otters around here, I am in the middle of built up east London area.
There isn't anything to take a photo of - just an empty pond now, plants all dried up and dead and chucked away.
It has been suggested that maybe birds were try to get at the insects in the plants.

Were these pond plants sitting on the surface or under water? Unless they were sitting with the pot proud of the water then it isn't small birds as they can't go underwater and given where you live I doubt you've had any ducks visiting the pond. It could just be mindless vandalism...

It may be youngsters fooling about.

or a fox that's taken to swimming...

mg is onto something I think .

The pots were proud and I had in the past seen a pigeon pecking at the gritted top - maybe they thought they were seeds [ if they ate much I bet they had a job taking off]

Many birds eat grit in order to be able to grind the food in their gizzard, pigeons certainly do. Normally with plants in ponds you keep the top of the pot below water level and then you don't have a problem.