Get a faster reply to a visa application process

I having been having issues booking a premium appointment for SET Long Residence for over a month across all the 8 centres in the UK. I try at different times in the day including at 12 midnight when it appears that new slots are being released. The form throws up different errors at midnight such as timing out, asking me choose a different date after selecting one that is free.

I know it is not just affecting me as I know of two people that have been trying to book the same SET LR appointment for 2 months and several people have been reporting this issue on this immigration board website

By chance, I stumbled across the get a faster reply to your visa application https://www.gov.uk/faster-reply-visa-application which can be used to get a decision for applications made via post within 5 working days for a fee of £510. I have read how it works (the postal application payment must have cleared and only the first 5 emails requests are accepted for the upgrade) but really want to find out if anyone has practical experience of using it or know someone that has?

I really want to find if getting accepted for a faster response is as difficult or straightforward (since it appears to be as simple as sending an email 8.00am on the dot) as my experience for booking a same day appointment.

Thanks for your help