Disinfecting Equipment

If the tank needs to be deep cleaned (eg it held fish with a very infectious disease and you want to make 100% sure) you can use dilute bleach - cheap supermarket basic range bleach which contains no surfactants or perfume, or baby bottle sterilising solution, all diluted 1 part bleach and 9 parts water. Rinse very thoroughly with water then a final rinse with water overdosed with dechlorinator.
The same can be used for equipment, though I wouldn't recommend bleach for a substrate; I would just throw that away.

But for most diseases, a thorough wash and scrub then allowing the tank and equipment to dry will kill most fish pathogens. They can't survive being dried out. But if you are thinking about lymphocystis, I admit that I did bleach the the tank and equipment that the infected betta was in to be on the safe side. I threw away the filter media and did a fishless cycle before getting the next betta. I'd never actually done one before!

The Tesco one is fine. And cheaper  The Milton is the same chemical, just a tad more concentrated (Tesco 1.5%, Milton 2%). But by the time you've diluted it they will be virtually the same.

And yes, the dechlorinator is to remove every trace of the bleach, which is chlorine.

You would have to remove every trace of copper sulphate before the plants went in your tank if you have any inverts (snails you want, shrimps).
Potassium permanganate should be OK but how effective it is at killing plant hitch-hikers I couldn't say.

I quarantine all my plants with carbon in the filter to remove anything that might kill my snails and shrimps. I got some bolbitis from MA a month or so ago, and I had tiny ramshorn snails and a bladder/tadpole snail appear in the tank. I've kept the bladder/tadpole snail; hopefully when it gets bigger I'll be able to identify what it is. It is still too small to tell if the whorls go clockwise or anticlockwise.
You could try quarantining any plants and maybe put something a hitch-hiker finds irresistible on the bottom of the tank to entice them out of the plants. Keeping the tank bare bottomed would get over the problem of any hitch-hiker hiding in the substrate.

I've never seen any swimming, they move by pushing head end forward and then pulling the rest after it...like a leech does. I did find 1 big one about  3/4 of an inch long when I first planted up the tank (my first ever tank) and naively thought that removing that solved the problem. I've never seen any on a fish and generally I find them on the underside of pebbles. Maybe I'm just being a bit squeemish about the things 

and Sue I'd also put the plants in water with a water conditioner for a while to make sure there was no copper left before I added them to a tank.

Funnily enough the none of the snails seem to have been in the least bit bothered by eSHa 2000 which is copper based. I took some assassins out of the tank I treated so I wouldn't lose them all but the ones I left just seemed to lock themselves down for the duration of the treatment. Maybe it'd be a bit different if my water was 100% conditioner free  who knows

I doubt a nuclear blast would clear the common snails from the tank  damned things get everywhere