Therefore you would have won...
£500!
Kind of hard to gadge this game.. If you had carried on, and not swapped, you would have had the £50k. But as you said you wouldn't have done that, you made a good decision. Though if you had hung on for one more round, you'd be that much higher up!!
But thats full of a lot of "What if's". Fact is, you got more then you would have done if you had carried on to the end, and swapped like you said you would have. So congrats for that!