So the s&p500 is basically tracking the north American version of the ftse 100. All in on equities in that case. No diversification there mate, aside from the other types of assets you can invest in, there isn’t even diversification geographically for the equities themselves. All or nothing in the US.
A really good diversified fund will give you a proper mix of assets and geographic spread and the increased management cost could be offset by the potential for enhanced returns vs a tracker fund.
Depending on how many years you have left on your mortgage and the outstanding balance, it would surprise you how much difference even one or two hundred quid a month can make in terms of overpayment.