There is one problem I can see.
If it's a Swivel Group 0+/1 then it should only be up to 4 years, not 12, because Group 1 ends at 18kg which is about the size of an average 4.5yo. If it was up to 12 years, then it would be Group 0+/1/2/3 - which part are you most sure about? The model name or the up to 12 years? Is the same seat model still in stock?
The other way to tell is that if it does convert for up to 12 years, you will see seatbelt guides where you would see them for a high back booster type seat, so at the sides where the child's hips would go, and at the top usually on the headrest or sometimes on the top of the side wings instead. These will be coloured red in an R44 seat (using the number Group system) or green in an R129 seat (which uses height instead).
I can try to find out which seat it's a copy of, where the manual might be easier to find. It is v tricky to find manuals for generic seats as the retailers often change suppliers as well so they aren't even the exact same model of seat over time. It's poor that their customer service can't provide you with the details. If you wanted to pursue this avenue, you could ask them who manufactured their own brand seat of this type in 2022, or try to find a manufacturer name somewhere on the seat, and then go direct to them. The usual generic manufacturers are Welldon, Team Tex, Eurotraders, or occasionally Joie (Allison Baby). I have a feeling John Lewis had a mixture of manufacturers with the 2022 range so it could be a wild goose chase. I do remember it being a topic of discussion on the car seat groups at the time and it wasn't a big brand like Cybex or Besafe - IIRC it was one of the generic b2b companies.
If you have a picture of the seat or any labels on the back of it then this might also provide clues 