Covering for smallish pots?

How about wire hanging baskets upside down over the pots?

Do you have prickly bushes in Manchester ? Do you have secateurs ? If so, you are sorted...

It's not quite that simple of course. But cats are inclined to find somewhere else to got to the toilet, and then stick with that location, if it is less...uninviting, than one that has prickled their a**e.
Ignore all that crap about protective barriers of eighteen inches to keep out carrot fly, you need to cover the carrots completely with fleece - no rips, no tears, no gaps. Personally I have in the past just draped fleece over the plants with string tied around the pot to hold the fleece on, and let them brush against it as they grow; if you want to be posh you can use some heavier gauge plastic coated gardening wire as crossed hoops to hold it off the plants, again tying round the bottom of the fleece.
I tend to use a Morrison's flower bucket with 8 - 12 holes drilled in the bottom for growing carrots in, with added sand in the growing medium; instead of fleece or Enviromesh from a garden centre, I use the insect netting panels that Aldi and Lidl sell every so often for covering windows and doors. Cheap, long lasting, effective, and in quite convenient multipurpose sizes for container growing.
If you have something white flapping about then birds are less likely to come near; white is an alarm signal to them. (I used angled stakes with streamers with great success one season.) But really, they will only come near netting if they see a ready source of food beneath it - eg strawberries, fruit, etc. I think the large mesh netting is worse for trapping birds, because they can see the fruit better and are more tempted, than 0.6mm anti-everything mesh.

I use hanging baskets to keep the cats off too

Nope, never had any of that in my pots

Maybe you're unlucky? Or overly worried?

I did have a squirrel build a nest in a pot one year using the gnawed-off carcasses of my other plants...

Never really had problems with birds or cats - I have a cat and can't imagine him wanting to use a pot as a litter tray. Too small, too high.

Nope, I think it is you who is lucky.

The cats around here, of which there are many, have often been seen curled up in a plant pot. Here's a picture I took a couple of years ago of my neighbour's strawberry plant.



My bluebells were seen and used as a comfortable feline bed a few times. One cat sat on my tray of chives. My neighbours solitary pot of strawberries was frequently used as a bed by one of her own cats. My raised bed was dug up in several places and used as a toilet until I fully secured it with netting.

My advice....if in doubt, cover it!

Three vertical canes taped equidistantly around a pot, then wrapped with net and tied at the top, should do it.

Good point, I hadn't thought of the cat bed angle.

We had a cat when I was growing up who killed a catnip plant...by sleeping on it. We dubbed it "catnap" instead.

I'm trying to train mine to sleep on the ground elder. So far no luck.

To be fair, there is only really one cat who hangs around here a lot (mine are indoor cats) and I think he's most likely to use them as a bed than a litter tray. Squirrels are another matter - it's squirrel centre around here (they are frequently on my window sill) - and Im sure they'd be happy to sit in my pots and maybe have a little dig around for some nuts whilst there. Add in a plentiful supply of magpies and pigeons and I'd rather be safe than sorry.

I think fleece over the top and tied around the pot with some string might be the easiest and cheapest option.

More squirrels than cats here. Every Spring a small forest of oaks and hazel germinate in the greenhouse bed where the dopey things have hidden them during the winter. They don't seem interested in small pots though, not when they can dig oodles of small pits in the grass instead. Then the moles throw up a molehill alongside to help me level it out. Wild animals can be sooooo considerate!!