If you have a hot-water cylinder, wrapping it properly is one of the quickest paybacks in the whole house. You have already paid to heat that water; an insulating jacket simply stops it going cold while it waits to be used, and the materials cost less than a couple of takeaways.
The bare or thinly clad cylinder
An uninsulated copper cylinder sheds heat constantly, so the boiler or immersion has to reheat it again and again. A thick insulating jacket cuts that standing loss dramatically, keeping the water hot for far longer between heat-ups. Even cylinders with a thin sprayed-on factory layer benefit from a jacket over the top, since the original foam is often thinner than ideal. The colder the cupboard the cylinder sits in, the more a jacket saves.
Choosing and fitting one
Look for a jacket of generous thickness, the chunkier the better, sized to your cylinder's height and girth. Fitting is a job anyone can do: the segments wrap around and tie or strap into place, leaving the controls and the immersion boss accessible. It takes minutes and no tools. If the cylinder is in a warm airing cupboard you use to dry clothes, bear in mind a good jacket keeps that cupboard cooler, which is the point.
Lag the pipes too
The pipes leaving the cylinder lose heat just as the tank does, so slip foam pipe lagging over every accessible length of hot pipe, especially the first stretch from the cylinder. The pre-slit foam tubes cost very little, push on by hand and cut to length with a knife. Lagging the pipes keeps the water hotter on its way to the tap and means less cold water run off before it arrives.
While you are in there
Set the cylinder thermostat to a sensible level rather than scalding, as covered on the hot water page, so you are not maintaining water far hotter than you ever use. And lag any cold pipes that run through unheated lofts while you are buying foam, since that protects them from freezing in winter. The whole exercise is an afternoon and a small spend for a saving that repeats every single day.