Even convicts said to have been minor thieves sent out to the other side of the world are British, they have family here, friends...therefore they would still be loyal, by not sending sociopaths they could colonise this whacking great lump of land at the bottom of the world and control the southern seas.
And that is what was really wanted, everyone focuses on the penal side of things but it was a great strategic coup back in the day, the British navy was second to none in the north and by having a powerful colony in the south they could really tie up the spice trade and have a handy stop off point...when transporting goods it makes little sense to simply have one ship going all the way to the eastern lands and all the ay back, far better to have many ships doing many journeys...
But I digress...
By having minor criminals sent to the bottom of the world, given a small tithe of land (I dont know if this is true or not, but it is how I would do things) and the promise of a new life you do get a rapidly expanding colony...isn't it great to have an abundance of criminals on your hands, be able to ship them off to a new land and make sure no one can get there through sheer force of numbers who, if not loyal to England anymore will at least still fight off any invaders of their land

I am repeating myself, it is one of my flaws lol
It was a win win situation, they get fewer criminals to worry about, they have this continent that they need to control