Sadly you don't mention what the model of the set you looked at Currys PC World is.
However, the Richer Sounds LG set you mentioned is a decent 43" model and if they're offering a service to disabled customers, along with a six year guarantee, I'd go for that. (John Lewis have the same model for £424 without any of the perks).
I think it's the LG 43UM7600PLB in the thread title.
What I would say is that LG potentially 'making the screen' for Sony doesn't mean as much as you may think. What this means is that the LCD panel is potentially the same, or made by the same manufacturer, across multiple models of TV, it doesn't mean they give you the same picture quality. That's because a lot of the picture quality is defined by the processing of the input signals before they get to the LCD panel. Even if they use the same SoCs - there can be some significant software and configuration differences that will impact picture quality.
This will be different between manufacturers - even if they use the same screen panels.
If at all possible I'd try and see both displays in person - and ask for the remote control so you can disable all the nasty motion processing, contrast enhancement, noise reduction etc. and compare them with the same content on both displays.
I compared LG, Sony, Panasonic and Samsung displays and could never get the LG and Samsung to look as 'unprocessed' as the Sony and Panasonic sets. This has been the case for the last 2 TVs I've purchased - though both were 49" (I wish I could go OLED but don't have the space for a 55").
My most recent TV is a Sony FALD LCD HDR set and is by far the best TV I've ever owned.
I'd recommend LG for OLED TVs, but am less keen on their LCD (sometimes called LED) displays. However other people see pictures differently and are very happy with their Samsung or LGs. To each their own.