1. It is 20th October.

2. It is October 20th.
3. It is the 20th of October.
4. It is 20th of October.
================
Which one is correct? Do they use only #1 in Britain?

I'm not sure that #1 is the only way a date is stated in the U.K., but it is certainly acceptable there. All are correct. In the U.S., #2 and #3 will be most common. #4 would be the least used, but it's not incorrect