Whether ex UK or ex EU, these "rocket bottom fares" never allow a stop over (or to rephrase, a stop over will always cost extra).

I'm not sure who told you that it's an ex UK sale and not an ex EU sale and to some extend it doesn't matter. They might still be launching marketing materials to class this as a sale ex EU countries but fare wise, this is in line with previous ex EU sales which tend to be always on when there's an ex UK sale. Having said that, there are some good ex EU fares that aren't part of any sale eg the CPH fares.

So, 2,100 euro is 1700/1800 roughly (he calculates from roof top in instabul looking at Asia and Europe). That's not a bad fare if it gets you to KOA and SFO and gets you the TPs you are looking for. I think historically, any stop over cost ~300 Euro extra. October isn't usually part of the "best fares seen" sales period.

So it's not an amazing deal and not what you wanted but unfortunately you won't get what you wanted. These things are hard to predict but I'm not sure you'll find a better rare between now and October when you are looking to travel. And as said, October usually isn't included date wise.

So if you can afford 2100 euro and want to travel on the flights you found, I'd book it. There's a chance a better deal might come along (and I've had that myself where I bought flights only to find I could have saved 200 quid a few months later - cest la vie). But I'd say chances of a better deal for an October trip to KOA and SFO with stop overs on the same ticket coming along are small.