This has been asked repeatedly. We cannot tell you what UKVI can offer at each moment. That depends on their capacity. The priority service is very sought after, UKVI will only offer it if they manage to process priority applications within their capacity otherwise only standard service is offered. This is made clear on the UK.GOV website:
There’s a limit on how many people can apply for a faster decision. If you’re not offered the option to apply for a faster decision, you can still complete a standard application.
https://www.gov.uk/faster-decision-visa ... ide-the-uk
Some people have reported that they see availability when they look at 1 AM, so you
might be able to book a slot if you tried at this time of the day
Advice is given based on my personal research and experience only. Do NOT contact me via private message for immigration advice