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FTSE Global All Cap has an OCF of 0.23%, so it might be cheaper to do it manually if you really just want something like FTSE 100 (0.09%), Europe ex UK (0.12%) and the S&P 500 (0.09%).
I guess it depends how much you want to sink into Japan, APAC and miscellaneous other (developing or whatever), which do have higher fees.
Whatever you do I'd just set it up once and leave it alone, if you're going to want to keep rebalancing then just choosing the global fund is probably more sensible.
Would it not be easier to just buy the FTSE Global All Cap Index Fund (Accumulation) instead of trying to manually diversify with a combo of LS100 and S&P500?