That would imply the decision making is happening outside of the locale. Why wouldn’t a firm just have their algorithms run loose on a machine as close to the source as possible?
Because you need to be close to the other venues where the symbol trades.
What's happening is arbitrage between New York and London, not decision making in London for New York listed stocks.
Various financial products have value impact on other products around the world. In order to price orders appropriately in a given region you need the most up-to-date price information available, especially if you are looking to avoid being on the receiving end of arbitrage trades. The same is obviously true for the arbitrageurs.