I am not an expert on this. But I would like to be ;-)
Here are some of the things I have come across. I'm sure there are others.
Taxes inside the fund; One thing to bear in mind, is that the taxes that will be charged inside the fund, reduce the return of the fund. Different domiciles will have different taxation agreements that they can use. e.g. the fund buys 1000 shares in a company in Brazil. the shares declared a dividend of 100 USD. Depending on the taxation agreement between brazil and the place of domicile of the fund, the fund might suffer different witholding and other taxes on this dividend. So one fund might get back 90 or 100 USD another might get back just 50.
Tracking error: The TER does not cover tracking error, it can be positive or negative.
I am of the opinion that the effects of the above are sort of systematic and repeatable, and as such we can take history to give us some predictive power over them.
My suggestion of how to see the effect of all these is to take a unit value of both funds on the same day in the past (ideally a few years), take another unit value of both funds today, calculate the return for each fund, and see which gave the higher return.
investors tax: It seems that the taxes due on ETF's are different for different ETFs. This could have a large effect on your personal effective return. As far as I can see, nobody knows the taxes due on individual etfs, people take a best guess and leave it at that for the moment. People's best guesses differ.
Currency: From a euro investor point of view, you are converting to GBP or USD when you buy or sell. I assume a similar exchange cost to/from Euro and GBP/USD. For a long term investor these two transactions will not have a very large effect. If you can use a specialised third party service to do your currency conversion you will save some money.
Hedging: Beware - read the prospectus. Sometimes funds hedge against currency movements. You may or may not want that. I think funds are more likely to hedge currency, when the fund is a priced in a different currency to the area the fund invests in. Comparing the returns of funds when one hedges and the others doesn't will not IMO give you any meaningful results.
My own view, is that I don't want currency hedging, it is a cost. I embrace the volatility, along with rebalancing, and hope it give a better correlation for my portfolio. I also acknowledge that most internationals already deal in a lot of currencies in their own bushinesses. I also invest in such a way that I am happy with the top level currency risk. (i.e. a significant amount invested in Euro based companies)
Loaning securities: Beware - some passive funds will loan out their securities with an aim of making more money for the fund. This increases risk, but also increases return. You should check the prospectus. Comparing two funds tracking the same index where one loans and the other doesn't will not give any meaningful results.