I have read that a "pony express" system was used for important government communication. Was this true across the empire? What about mail between regular citizens?
27 days under ideal conditions (meaning summer) if you travel from Ostia to Narbo (present day southern France) then follow the mountains west and then again by ship north to Londonium. This is also the cheapest route for a passenger (although for transporting goods the sea route through gibraltar is cheaper).
There is a super cool tool called ORBIS developed by a couple of Stanford historians. It's basically google maps for 1st century rome. You can change around a whole bunch of cool things like season, route, goods vs people etc. http://orbis.stanford.edu/