Trying to do the flake thing is usually a mess for beginners (not to say it doesn't work for old experienced folks, but..) In the UK, the places we hear the most are Boots, HomeBase, possibly Robert Dyas, possbily B&Q.
I think the Boots and HomeBase sources and the kind of plain brands there seem to be the most reliable for the UK folks and there's at least one type of Jeyes Kleen-Off that's ok but that brand seems to be one that has a whole lot of "flavors" of ammonia, some of which have the tnings in them we want to avoid.
To repeat the frequent advice: it shouldn't have dyes, fragrances, surfactants or soaps and the one test you can sometimes do is to shake it (if you can see it) and make sure the bubbles go away in 2 or 3 seconds just like water. You do not want it to foam after shaking.
If it tells you want percentage (9.5% is common in UK since there's a rule that it not be 10% or above) marked, don't just believe it and plug it into our calculator and dose.. instead be a little skeptical, start the dose a little lower and test your way up to a good 3,4,5ppm looking test result, making sure its not 8ppm.
~~waterdrop~~