Greatoaks
Yep that's the way it works, the system will allocate a seat. You can for a fee select a seat, if there is more than one of you travelling it will not try and split you up, but you might be on a different row if your seat was the last on that row, it will normally start from the back as people who pay for seat allocation normally want to sit near the front, bmibaby use air-bridges at airports that have them so being near the front means you off quicker, but if your travelling with luggage it means you'll get to wait longer for it!
Unless the flight is full the cabin crew will always try and move people who wish to sit together but have been allocated seats apart.