We are coming to London June 11, 2012. We are considering buying a 7 day travel card, but are making a few trips out of zones 1and 2 and are wondering if a pay as you go oyster card would be better. We will be there until June 16. We are going from euston out to watford junction June 13 around 8:30 and returning that same afternoon. How much is it to add on to a travel card for zones 1and 2 to go out there?