Christopher Wren's Monument to the Great Fire of London (The Monument) is 202 feet tall.
202 feet tall
Hertfordshire is directly north of London, about 50 miles from the centre of town. It depends on where exactly you start and where you want to get to, but would probably involve a train from Euston...
Big Ben
If you mean how far, then it's approx 100 miles.
London is located in the southern part of England.