As a lawyer you will of course be able to read the applicable Migration Regulations from which you will note that what is required is a positive skills assessment. The legislation does not explicitly require you to be admitted as a lawyer in Australia before you can be granted a 189 visa, but does require you to have a positive skills assessment, amongst other things. So, really the question is whether you can get a positive skills assessment without having been admitted as a lawyer in Australia. You have mentioned that you intend to apply for a skills assessment from either Queensland or New South Wales. On the Legal Profession Admission Board of NSW website,it says:
Therefore, in order to obtain a "Skilled Migration Letter", a person will need to have overseas qualifications assessed in accordance with the Board's Rules and the Uniform Principles for Assessing Qualifications of Overseas applicants for admission and then be admitted as a lawyer.
I think you are very unlikely to get a positive skills assessment from NSW without having been admitted as a lawyer in Australia, and I have heard that QLD is even more difficult than NSW, but the NSW Legal Profession Admission Board's advice on their website (quoted above) is not an accurate statement of the legislative requirements set out in the Migration Regulations. That said, it won't help you obtain a positive skills assessment from them, which is what you need. If you can obtain a positive skills assessment from a State Law Admission Authority in Australia based on your current qualifications and experience, then admission in Australia as a lawyer is not a prerequisite to applying for a 189 visa.
For the 189 visa, you will see from the regulations that there are criteria to be met at time of application and at time of grant. The regulations do not provide for meeting the criteria within a timeframe after grant.
In order to be admitted as an Australian lawyer, you will be required to complete some academic subjects and also some PLT (practical legal training) subjects. For the academic part, I am not aware of any possibility of sitting the exams in London and certainly not online (for the exams... though study online may be possible), but for the PLT part, I think the College of Law does have the option of sitting the PLT exams in the UK.