Winemaking frequently starts off in a bucket, but usually only for the first week or so of vigorous fermentation. After that, when fermentation slows down, oxidation becomes a problem. A bucket is fine if it has a tight fitting lid while CO2 is retained in the fermenter, but once you start taking the lid off to check SG etc. that protective head will be lost, and then the surface area exposed to air needs to be reduced to a bare minimum. When winemaking can take several weeks or even months, that's why it should be in a sealed fermenter under airlock, and topped up right to the neck.
Glass demijohns aren't strictly necessary, and can be quite expensive if you have to buy new, so use Freecycle or check around the car boot sales. 5 litre plastic mineral water bottles are ideal, and should cost less than a quid from your local supermarket.
Oh, and make sure you get the Original Ribena which is loaded with sugar, and not the diet-friendlier version which is loaded with artificial sweeteners.