Most West End audiences are mixed - locals, tourists, school groups, first-timers. The unwritten rules below are not about being formal, they are about not getting in the way of other people who paid to see the same show.
When to arrive
Aim for the venue thirty minutes before the start time. Bag checks are now standard and bars at the larger theatres queue heavily before the curtain. The auditorium opens around fifteen minutes before curtain up.
Dressing
There is no dress code at any West End theatre. Smart casual is the safe default. Audiences at opera and ballet trend slightly smarter than at musicals, but no venue will turn you away.
Phones
Off and out of sight. Most ushers will quietly tap your shoulder if a screen lights up. Photography of any kind is not allowed during the show.
Drinks and snacks
Most venues now allow drinks in the auditorium in plastic glasses. Wrapped sweets are fine. Crisps, anything in foil and anything from a paper bag tend to annoy the people sitting near you.
Talking
Reactions are fine - laughter, gasps, the spontaneous applause when a famous song starts - but stay quiet during dialogue. Resist the urge to sing along during musicals; the cast is paid to do that.
Standing ovations
They are common at musicals and rarer at plays. Stand if you mean it; you are not obliged to stand because the row in front did.
If you need to leave
You can step out for accessibility or urgent reasons and ushers will usually re-seat you at a suitable break. Re-entry mid-scene is at the venue's discretion.