I arrive on BA in biz class, connecting to OU (Croatia Airlines) in biz class next month, all on one ticket issued by BA. I am BA Gold.

I have a 5 1/2 hour layover. Croatia uses the South Terminal at LGW. I have two main questions.

First, is flight connections where I stay airside still a thing (it seems it was in 2017 when this thread was started)? The LGW website indicates that this is basically a landslide connection. If I do have to go landslide, are there any restrictions about when I can go through security? In other words, will I have to hang around the terminal until check-in opens for my second flight?

Second, can I use the BA lounge as a passenger arriving on a BA flight, even though my onward flight is on OU? The lounge OU uses is pretty poor, though better than nothing. I also have Priority Pass if BA is not an option.

Thanks in advance for any help or experience you can share.