Which class are you traveling in? OU are strict with hand luggage and so are the ground staff at T2, you may not be able to take rollers aboard.
Also I would emphasise what is said above, if you access T2 on transfer you won't be able to leave again.
The Croatia evening flight to ZAG is usually significantly delayed too.
I would store the bags at left luggage in T2 - take the tube from T5 - it's free and just pay to leave your bags there, there isn't a better option as unless it's one ticket BA won't check your bags through.