My name is not Maggie. Sorry.
The average day rate is between £115 and £150. All the big productions use standard BBC / Pact rates and will tell you what the rate is before you agree to do the production or not. You get extra for having a haircut, overtime etc etc.
https://www.wegotpop.com/pages/uk/background-artist/rates/pact-faa/2023/
You don't have to have your own transport but it makes life a lot easier if you do. Last call I had was for 7am on a Sunday morning at an industrial estate - not easy with public transport and if you're paying for Uber etc then that's cutting your earnings. Quite often if the filming is in a city centre then the production base will be somewhere else, you'll meet there, and be bussed to location.
Have never done an audition. It's important to remember that you are NOT there to act. You are just there to fill in the background or walk along a street, for example.
When you register with the agency you fill in a LOT of information about your ethnicity, piercings, disabilities, physical appearance. Depending on the production they might be looking for a particular "type" - Afro-Caribbean women aged 25-35, or east Asian clean shaven men with piercings. For the more general castings they are just looking for a mix of people of different ethnicities.
You do pay the agency and the way they do this varies between companies - all laid out in the terms and conditions before you sign up. One agency I'm with takes £25 as a one-off annual fee then 15% off everything you earn. Other doesn't take an annual fee, but takes 20% of earnings.
Haven't done anything particularly weird yet - but worked with a guy who was an extra on a battlefield scene in Outlander and spent the day lying in a ditch pretending to be dead. The glamour.