Regents Park is in London NW1. Regents Park London N3 is Regents Park Road in Finchley, London N3. If its the road in N3 Liverpool Street is marginally easier. Take the Circle or Metropolitan Line to Moorgate and then the Northern Line northbound to Finchley Central. Starting your journey from Southend after 09-30am weekdays or anytime weekends you can buy a One Day Travelcard, the price of which which includes one return train journey to London plus the freedom of the London Underground/bus network for the day
www.nationalrail.co.uk