There are already some proper advises and analyses by georgi and internet human.
Just adding my 2cents in the form of a questionnaire:
Why did you get started doing sound in the first place?
What has kept you interested over time?
In what way are you different than all the others?
Do you need to be in London to do this?
What do you need to feel confident ...
looks like post houses hire runners and promote them internally when they need. yes if they're unpaid it's unethical, but there are tens of reasons in support of this too, and it seems to be the preferred way of screening candidates for full-time positions.
occasionally houses like Radium offer sound designer internships (again, unpaid), at least they're ...