It's a gamble. If you want one of the five deaneries which include London, Oxford, Bristol, Birmingham or Manchester, then you might not get your first choice. In fact, in the case of London, you are more likely to not get your first choice and some years the Oxford and Bristol deaneries are not far behind.
My dd applied in the second year of the random number allocation system. She really wanted London as that is her home city but decided it was too much of a risk, so went for one of the other oversubscribed deaneries (where she had been at medical school). She got her first choice but the friends that she had intended to share a house with didn't.
As pps have said, even if you do get the deanery that you want, the same process then applies for the locations/jobs within that deanery. Scotland and Wales are both single deaneries so the distance between where you want to be and where you end up can be huge. And there are the anomalies of the Isle of Man, the Isle of Wight, Jersey etc within all this which she was keen to avoid.
When it comes to choosing the actual jobs in the second phase, she didn't get the job that she wanted and was initially disappointed. But she has loved the placements that she thought she wouldn't like and not been quite as keen on the one that she thought she would love. She has also managed to swap her second year with another doctor for something more in line with what she originally wanted.
Overall, all you can do is cross your fingers and hope for the best!