There's signs and posters encouraging 'seat holders' to offer their seats to pregnant women, the disabled and elderly people.
The first problem with this theory is deciding who is eligible for the 'disabled, elderly & pregnant' category.
What's the tipping point between being kind and rude?
How do you know what age is appropriate?
You might offend a old looking 50 year old...
Or even worse, mistake a fat woman for being pregnant.
Where are the guidelines?
If someone has one arm, does this make them more worthy of your seat?
I'd rather avoid these situations by standing.
If you do end up sitting the best technique is to keep your eyes down. Never look up, then you can't get it wrong. Even better, make sure you pick up the Evening Standard - then you're clearly to engrossed in your newspaper to notice the frail 80 year old collapsing directly in front of you.
Problem sorted!
