Damn this is going to be complicated. I should just pay a professional :)
That's a joke, right? You should not be paying a professional for this. The work that you'd need to do to make sure that a professional has done it right is more than the work to do it.
Below a certain level of dividends and Capital Gains Tax (CGT), you don't need to tell HMRC anything; above that threshold you need to tell what you've done, but not pay tax; higher still and you'll need to pay tax. It is absolutely in your interests to max out the ISA allowances available to you.
If you invested £100k outside of an ISA and it gained £11k (not impossible), then you'd need to manage CGT after the first year.
Another thing you should look into is CGT harvesting. You don't need to do it before you start, but if you are buying income funds rather than accumulation funds it will make it easier (ignore people who say that accumulation funds mean you don't have to worry about dividends - they are well meaning but wrong - no-one on this thread, but I see it now and again on the boards). Set a reminder to read about it in February 2019, the Monevator article is good.
Does this hypothetical person have a trusted spouse or partner? What's the housing situation? LISA is nice if you're planning to by a first home at some point.
Also look into the treatment of bond/equity funds. You can hold bonds in a mixed fund (like the Vanguard Lifestrategy funds), and it is treated as dividends rather than income as long as there is a sufficient share of equities in the fund. I recall it is up to 40% bonds, but DYOR.