Could you tell me how do the British express that they are madly in love? Thank you)
In the same way as the rest of the world - we're not an alien species.
Well-know British films showing aspects this theme: 'Four Weddings and a Funeral', 'Love Actually, 'Truly Madly Deeply' etc etc etc
I fancy you! A lot! Does not mean they are madly in love. You can fancy someone a lot even if you do not know them!
I fancy you! A lot!
"I'm madly in love with you."
Maybe you could look at Shakespeare's sonnets for some really extremely poeticized and timeless sort of examples, but people don't actually talk like that, since it's really old, all spoken as poetry, and also since he was specifically known for doing nonstandard things with English for poetic effect.