Can you do the EES stuff in advance?
We're travelling at the start of August and I'm aware this may not be known yet as it's a new thing but - we're arriving in London the day before our morning Eurostar, and it occurs ro me that it might be a good idea to pop over to STP in the evening and do all the biometric stuff so we're not faffing around with it at the same time as everyone else at what's bound to be a busy time (9am, first Monday in August).
Or is it something you have to do at the same time you actually go through immigration?