@johnboy99 This will take a while to answer but I hope you will read through it all.
1) The FTSE Global All Cap Index Fund is the most neutral, multifactor, passive fund out there. By choosing this fund you are effectively saying thank you to every active institutional/ retail trader, money manager, hedge fund, options trader (etc). These people are collectively setting the price of every stock and if you believe in market efficiency (you should), it's the perfect fund.
2) The above fund is what the average investor should choose.
3) Developed world ex UK is taking a position. You'll be excluding small cap stocks, UK stocks and all Emerging Market stocks. Why should you do this? The answer from a rational/logical side is, it's a bad idea. Perhaps, avoiding the higher fee fund is the most logical way of looking at it, but your post seems to be hinting that you want higher returns rather than a 0.09% lower yearly fee.
4) Where do stock returns come from? The answer is "expected" stock returns (using discounted cash flow models) and "unexpected" stock returns (you can't predict this one). Seeing as you can't predict the stock market (trust me, you can't), it's foolish to look at historical returns from the two funds and use it to predict future growth (you can't invest in a back-test). There has also been a lot of unexpected growth in large US companies in the past 10+ years, don't let the past cloud your current judgement when it comes to your money. Again, unexpected means unexpected.
5) When your knowledge on investing increases in the future (look at factor funds). You are far more likely to sell/switch the Developed World ex UK vs sell the Global All Cap Index Fund.
6) This is anecdotal: During the tech wreck of 2022, the UK stock market was one of the best performing indexes from around the world. Don't always assume that a higher allocation to the USA creates guaranteed out-performance. Diversification is your friend.
7) Neither of the two funds are a "bad fund". I've made my position clear that the FTSE Global All Cap Index Fund being better out of the two, but the other isn't necessarily a bad fund. Investing should be boring and picking a fund which is completely neutral to your own personal biases is probably a good thing, as you will be wrong.
8) Lastly I keep saying things like "you can't predict the stock market", "trust me, you can't predict it", "you will be wrong". Don't take these as insults. Fully knowing and understanding that you're completely inferior vs the hundreds of thousands, possibly millions of experts/institutional traders participating and competing against each other in Global Markets is actually the first step to becoming the best investor. Thinking "its easy, I'll just exclude small caps, UK stocks and EM stocks and I'll produce alpha returns in the future due to historical returns" will get you laughed out of any room with financial experts.