"Recipe" for home-made SmoothOver?
Pre mixed joint compound, ( UK vendor) either a low dust mix or a topping compound, is a LOT cheaper than a tube spackle.  One 'recipe' is to add a bit of water to make it creamy, like your Polycell product.

It still requires sanding.  Invest in a few knives (6, 8 and 12" wide blades), a pole sander and some sanding sponges. A hawk and a bucket scoop are optional, but very handy.