Tube use is now included in the Admission price. The only other expenses to possibly incur is Locker Rental and Food Purchase and neither is required.
All lockers are all day lockers.
Key rental will be at the front gate. You may purchase your key when paying admission. The key must be returned at the front gate. This is your key for as long as you are in the park. Season pass owners must still return all rental keys at the end of the day.