Sacks are cheaper than the blocks. I have sacks of "tablets" or "pebbles" delivered in bulk, the 25kg sacks rather heavy, but you can get 10kg sacks. For example www.wickes.co.uk/search?text=salt You have to be able to lift them up to tip into the top of the salt bin.
Looking back, I bought 10 25kg sacks in March (about £60, delivered) and still have I think two left. At the time, 10kg or 15kg sacks must have been unobtainable, or perhaps more expensive. Store them in a clean, dry place, away from any metal tools, car, lawnmowers etc as any trace of salt will corrode them. It will damage your hoover so use a dustpan and brush for spillages.
Be careful not to get table salt, or rock salt, or road salt. The stuff compressed into tablets is fairly clean to handle.
They are also sold by catering suppliers such as Bookers because they are used in commercial dishwashers and glass washers. My windowcleaner has a softening plant and drives around with a tank of purified water in his van, and a pump, because it is so much better for streak-free cleaning.
You use about half as much washing powder, need no fabsoft, and can clean your bathroom and windows with a sponge and a bucket of soften water with a bit of WUL. You do not need to buy limescale cleaners.
If anyone in your house wet-shaves, they will notice the difference immediately.
There is no saving on gas or electricity.
Your washing machine might possibly last longer, I haven't seen evidence. Your towels will be softer and your hair shinier, not straw-like.