At least here in the U.S., the minimum ready-mix (delivered in a truck) is about 2 cubic yards. If you only have need for a small amount, you can get cement in a bag from your local DIY (at least here in the U.S.) It's all ixed up with the sand, garvel and cement, you just add water. There's about a cubic foot per bag... it's relatively inexpensive, but heavy... maybe 60 pounds per bag. You can get several varieties... such as quick set and hydraulic cement...
I take it that you mean ready mixed concrete....How much do you need and what for?.....Companies like Wickes or B&Q sell 10kg bags that you mix with water.Be brave and get sand and cement and mix it yourself.....six parts sand/ballast to one of cement...much cheaper.
There are small bags at Wickes. It's actually a gauged mix, with the cement in a small bag inside. Just empty out and mix together .. not too wet .. add water slowly.
I've used post mix for small areas, you need to put the water in the tub and use it quickly, but it's tidier than having to use a shovel to mix on the floor.
It's a ready-mixed powder cement that sets in around 20 minutes after mixing with water. I keep a tub of it at home to carry out repairs to the garden paths as it's as strong as a normal sand/cement mix. The tub contains two individual sealed bags and each bag has a shelf-life of around two years after opening. There's really no need to go fiddling about with small bags of pre-mixed mortar.