So, Lyles is not invert? I was just reading about sugars to add to a bitter. I went with table sugar but thought I should get some Lyles next time.
Golden syrup is a roughly 2:1 blend of lightly caramelised invert and non-invert sugar. I shared a video from Ragus about its production here : https://www.homebrewtalk.com/forum/threads/golden-syrup.655474/
It was widely promoted by some of the early British homebrew writers not as The One True Substitute for invert, but as a readily-available "hack" that at least was closer to invert than white sugar. Since syrup can be found in any British kitchen and is literally sold side-by-side with sugar in any British supermarket, those old writers were simply saying "since you have the choice when shopping in the UK, buy this rather than white sugar, it's a bit closer to what really gets used". However the hacks intended for a local audience then transmute into gospel when they go overseas...
Syrup does have a distinctive taste, although it's delicate enough that it would be swamped by the buckets of crystal that US brewers seem obsessed with putting in British styles.
I see lots of AIH (aka homebrewing.org) recipes call for Lyle's Golden Syrup. Substitute for an all grain brewer?
Well those recipes are trying to emulate the kind of recipes used by the big British commercial brewers in the mid-20th century, who were not all-grain brewers. However that has changed a bit - under pressure from CAMRA some have moved from using adjuncts in everything to saving them for speciality brews, and it's benefited the beers. But 10-15% brewing sugar is pretty "traditional" in 20th century British commercial brewing.
So to that extent, you just drop it and replace with more base malt. But bear in mind that sugars act as "anti-crystal" to some extent, so if you lose the sugar then you want to reduce the amount of crystal as well. For instance, Fuller's are now all-grain on their main partigyle that makes Pride, ESB etc and use 7.2% light crystal, which is about as much as I'd want without any sugar to balance it; conversely the famous Boddington's of the 1970s was about 10% adjunct with zero crystal. That gives you an idea of the extremes, but a good rule of thumb when you're starting out is to use as much sugar as crystal and then adjust to taste. But these US recipes that have 10+% of crystal in bitter are just nonsensical.