Don't tell them that you are just looking. Decide on a story in advance - that you will be moving for work in July for example - and call away. Don't mention cats until you are there, otherwise many will just say that they can't accommodate this and won't show you around. And you must certainly not pay to look at flats - they are being paid to find tenants, and don't need paying twice. If you tell them you are flexible about the moving date and are really waiting until you find the right place to live, then there will be more for you to see.
I would say that, as this is just a scouting trip, use the bigger agencies who can afford to have their time wasted, not the smaller agencies. So, try Foxtons, who are particularly evil and have a lot of properties to show!
Where in North London are you planning to move to? Or is this the point of the trip?
Vicky