Not in the UK but wanted to share my premixed lye water story.
My very first soapy experience was a workshop that used premixed lye water. The instructor used a totally different way of calculating the recipe from what I now use based on
lye calculators. (I went to 2 soap workshops by the same instructor.)
The recipe was for a 200g bar. It called for:
Hard oil: 100g CO
Soft oil: 5% of hard oil which is 5g (we had options of OO, sunflower, rice bran)
Additives: 1% (Oatmeal, EO, cocoa powder, etc)
15% Lye water was from this supplier -
https://www.theearthelement.com/product/sodium-hydroxide-10/
The soap turned out to be soap. Traced and gelled. But after that when I did my research, I couldn't understand how the workshop used a 15% lye water since I use at least double of that strength.
Months later when I made my own soaps using
lye calculator recipe, I spoke to the instructor and she was very surprised my soap turned out well. That made me wonder for a while whether there was a "secret" hidden way of making soap? She mentioned the "western" way of making soap is different since we're in a different climate in Asia. I was thinking like whuuut?!???
Anyway, I wouldn't buy premixed lye water. If I had bought from that website, I would be spending more than 20x the amount I would spend buying dry lye beads and distilled water. (I mix mine in the shower so I can spray over the floor afterwards.) I was initially put off soap making by the cost of supplies the instructor recommended. But I did my research and here I am, 11 months later, with more soap than I can use.