To be honest I have no idea how much a pair of spectacles cost in Croatia, but they are not exactly expensive over here. Specsavers have them from £25. Personally I would feel more comfortable buying them in this country. If there was ever an issue it would be easy to pop back to somewhere local. Also I feel I would trust a British opticians to get the prescription right, and I'm not sure how I'd feel about that abroad.
I'd suspect that the ability of the Croatian opticians should be fine and quite possibly cheaper (but that is pure guesswork on my part) but my concern would be if there was a problem and you had to get an apology and wait for a replacement pair, are you there long enough to wait for the replacements to become available ?