As RedBaron says, all the message means is that they are not able to make a decision in the time allowed. You should be "worried" in the sense that you will not receive a decision in the Priority Service timeframe. This may impact your travel plans. It does mean your case is not straightforward, but it does not mean your application is going to be ...
I have been in a similar situation before. Make sure you submit the following to justify your financial stability
Salary Slips of last 6 months
Income Tax Returns of previous years
Mutual funds/Stock statements/FD statement etc.
In my opinion, you should also write a cover letter explaining about your finances where you can mention about ...
One of the big hurdles for a PLAB candidate is convincing UKVI that the candidate will leave the UK after the examination. To that end, UKVI will examine the strength of the candidate's ties to the home country. The stronger the ties, the more convincing that the candidate would indeed depart the UK as the visa requires.
(UKVI will also assess other aspects ...
You should book for ten days.
The How to quarantine guide from the British government states the following:
"You must quarantine in one place for the full quarantine period, where you can have food and other necessities delivered."
"You are not allowed to change the place where you’re quarantining except in very limited circumstances...&...