Ashamedly I know nothing at all about ISA's or what the "FTSE Global All Cap" is
It is just giving them your money to invest how they think is best?
What kind or returns can you expect? - I guess that depends on the option you choose and how much you put in, but if it's going to be like £10,000 to get maybe £300 a year then it doesn't seem worth it
I'm no expert myself, but co-incidentally it's been something I've been looking into too

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Each tax year you're able to deposit 20k a year into an ISA (including Stocks and Shares ISAs) , the benefit of which is that you then pay no capital gains tax on any gains made. This can be useful if you invest, or gain, substantial amounts over the years.
"It is just giving them your money to invest how they think is best?"
Pretty much. There are different funds with varying risk profiles, but the "FTSE Global All Cap" is an example of a broad fund that ticks a lot of boxes. Another is Life Strategy, some have more emphasis on UK companies, others US etc. The fund I've been looking at an an accumulation fund, so effectively any dividends from companies you hold shares in are reinvested. The compound nature of this over time apparently is responsible for a lot of your gains, so you don't need the index itself to be sky rocketing as such.
Of course though, if the stock market has a dire time you could potentially lose money too (though it pays to remember that you will have only lost money if you cash out, if you're leaving it for years you could just wait it out). From examples I've looked into, due to the bullish nature of the stock market over the past several years, you'd have more than doubled your money if you invested it in something like this 10 years ago. Of course the past may not be indicative of the future, so the general consensus appears to be that the longer you just leave your money in there and don't react to events, the better off you'll be.
Being that the stock market has all but fell off a cliff of late (but has staged a partial recovery), it seems like it could be a decent time to invest (well certainly compared to a few weeks ago!).. It could of course drop much further over the coming months, but at the same time over the long term maybe that'd be a minor blip in the scheme of things.