For what it's worth, I've also had some problems using my Chase Sapphire Preferred in Japan in the year I've been living here. Some places take it, some don't. Of course, Japan is a pretty heavy cash-based society, so it may be more of an issue with the country not being as familiar with credit cards as it is with the CSP itself.

While I'd definitely prefer Chase points to Avios points any day, it is nice that the BA card will give you 1.5 for every $1 spent.

My suggestion would be to get both. Use the CSP whenever you can to accrue Chase points and transfer them to United miles (since that is your preferred airline) and when that doesn't work, whip out the BA card. I'm sure you'll find a way to use both types of points, as BA points can be handy, even for intra-Europe trips that can sometimes be expensive.