The price of a One-Day Travelcard varies, depending on the time of travel and your location. (London is split into 6 zones, with Zones 1 and 2 being Central London and Zone 9 on the outskirts of London.) To travel from Watford (Zone 7) to Baker Street (Zone 1) will cost an adult £11 for an Off-Peak One-Day Travelcard (travelling after 9.30am Monday-Friday), or £18.60 for a Peak One-Day Travelcard.
1 Additional Answer
The cost of your ticket will depend on which Zones you need to travel through. Off-Peak One-day Travelcard used in zones 1 and 2 costs 5.60 for adults, zones 14 costs 6.30 while zones 16 costs 7.50. Peak One-day Travelcard costs 7.20, 10 and 14.80 for zones 1-2, 1-4 and 1-6 respectively