Those dates look correct to me, although I believe the UKVI website only has appointments available 42 days out, not 45 days... they've changed the instructions from 42 days to 45 days, but don't seem to have actually changed the appointment booking service yet!
Earliest date you can attend an appointment = 6th May 2018
45 days before May 6th = 22nd March 2018
42 days before May 6th = 25th March 2018
- If you tried to book an appointment tomorrow, the website will only show you available appointments up to 25th April (42 days from tomorrow), which is too early for you to book.
- If you tried to book an appointment on 22nd March, it would only let you see dates up to May 3rd... which again is too early to book
- If you tried to book on 25th March, it would only let you see appointments up to May 6th... which is okay to book, but you would only be able to book for May 6th and no other dates.
In order to be able to see available appointments right up to June 2nd, you would have to wait until April 21st.