Playing with Fire, Never Give Up, Daverobev - thank you one and all. Very helpful indeed. Exactly what I was looking for - some debate, pointing out a few flaws and a jumping off point to go away and do some of my own research and thinking.
I've done just that this weekend and explored Monevator (paying particular interest to the article on Vanguard LifeStrategy) and the UK Tax thread on this forum where there are subjects that relate to my question.
Without going into too many details, some info about me and the investment:
I opened the S&S ISA with Vanguard on 10 March 2020, buying £500 of the FTSE U.K. All Share Index Unit Trust - Accumulation (0.06% ongoing fees) and £500 of U.S. Equity Index Fund - Accumulation (0.1% ongoing). Since then I have drip fed in small amounts to both. I now have about £2,500 in the ISA, with 59.7% in the UK and 40.3% in the US.
I have also transferred an old work pension from Aegon to the Vanguard SIPP. It is almost £10k. I have yet to allocate funds and the transfer has just completed this weekend.
I am 38 years old. These are my only investments as I have travelled the world extensively and saw that as my investment previously - an investment in life if you will. I have no dependents and no mortgage. Essentially, I'm working, I'm frugal by nature and I can kick in a high percentage of what I earn every month to my ISA. I've always been interested in the financial world and after landing on Money Mustache and the JLL Collins 'Simple Path...', I want to invest for the rest of my life. I'm happy with big risk and will stay 100% equities for the time being as I feel i need to catch up. I will reassess that in 15 years or so. I have yet to figure out all my figures for when I want to be FI etc, but that is to follow. For now, I am just keen to get started.
This is where I am at:
LifeStrategy looks so damn appealing for the simplicity, a bit of home bias and decent fees. However, one question. If I go LS100 and want to balance risk at a later point, down, say, to LS80 or under, how do I go about that? Is it is a case of just shifting from one pot to the other? Whereas if I am invested in 100% equities in a few funds of my own on Vanguard, I can then balance risk by just buying some Bond index funds to make 20% of portfolio and up etc.
Also, with LS if it extremely passive (an advantage and disadvantage) as I'm very interested in learning about investment and I find the whole area fun so will likely be reading more and more. And, with that in mind, I maybe would prefer a more active role in my passive portfolio.
I take all of your points about not being too overexposed to the UK and also the point about a small number of companies making up a large part of the UK index fund i mention.
So, I'm leaning now towards, having the FTSE U.K. All Share Index Unit Trust - Accumulation that I already have, ditch the U.S. Equity Index Fund - Accumulation (although, rather than switch this into another fund, I'll just leave as is to just do its business - unless I should switch out?), buy FTSE Developed World ex-U.K. Equity Index Fund - Accumulation (ongoing fees 0.14%), and buy Emerging Markets Stock Index Fund - Accumulation (0.23%) to give some exposure to those markets such as China, India, Brazil etc.
So, three funds, to keep it simple(ish). UK - 20%, World - 75%, EM - 5%.
Am I far wrong here?
Am I just been silly and should really just be bunging all the money so far in the LS 100 (fees 0.22%) or FTSE Global All Cap Index Fund - Accumulation (fees 0.23%) and all future money into the one pot?
Thanks for all the interesting pointers so far.