"Available from" plus date meaning?

Solution 1:

You should assume the vacancy begins at the beginning of September 2019. It is possible, but unlikely, that they mean the vacancy has another specific start date within September. If the latter were the case, they would have normally stated it.

Speaking as an employer, I am in most cases willing to wait a little time for the right candidate. Therefore, if the best candidate says, for example, "I am only available from 19 October," as an employer I will in most cases agree to wait until then. It will depend on how strong the candidate is in comparison to the others.

In practice the best approach is to go to the interview, and see how it goes. If you are successful, gently open up discussion afterwards regarding potential starting dates. You might be surprised how flexible they are.