Which places are better to work as a waitress? (UK)

pubs, restaurants, casinos, clubs...?

I'm really looking for a job with flexible hours (I'm a student)
being a waitress is the only job that comes to mind...
are there any other flexible jobs suitable for a student?

You could try the big companies like Zizzi's, pizza express, nandos etc. Or more even smaller restaurants. Chain pubs are also a good bet or if theres a shopping centre near you, the shops will be open weekends and some are open late night as well. Fast food restaurants are also an option MacDonalds take on a lot of people and are VERY flexible with their hours. Pubs like you say and university on site shops/bars/eateries only employ their own students.