I did the route in Sept/Oct 2009 and loved it. There were only three or four days of rain but foggy much more often. I used three sources of information for route planning, a book, Bicycling the Pacific Coast by Kirkendall and Spring, a pamphlet, the Oregon Department of Transport map for cycling the coast, and the ACA maps. I only purchased the ACA maps for California because I was not impressed with the route ACA chose through Washington and the ODOT map for the Oregon Coast gave me all the information I needed. For California I found the ACA maps more detailed and more up to date than the book and would highly recommend them.
Kirkendall and Spring state in their book that they averaged 53 miles (85km) per day to do the trip from Vancouver to San Diego, which worked out to 35 days of cycling. That should give you an idea of how long it should take you. The route is not particularly difficult. There are very few serious hills, most in Northern California, but there is a lot of short up and down hills that can be a bit of a challenge over a full day, mostly in Northern California and Oregon.
Below I have listed some useful links for planning purposes.
ACA Maps online: http://tsteven4.qwestoffice.net/
ODOT Maps: http://www.oregon.gov/ODOT/HWY/BIKEPED/maps.shtml#ODOT_Maps
Washington State Parks by Region: http://www.parks.wa.gov/parks/
Oregon State Parks by Region: http://www.oregonstateparks.org/searchpark.php
California Parks by Region: http://www.parks.ca.gov/parkindex/
On the ODOT site there is a link to Greg Current to get a hard copy of the maps. Try and see if you can get them sent to you. They sent them to me in Canada so they will probably send them to the UK as well. The coast map is very useful and I would recommend most of the alternate routes with the exception of the Seven Devils route that I took and was not impressed with and the Three Capes route which just looked too difficult. Maybe on another trip I will try it.
There shouldn't be too much problem getting into a park in Washington or Oregon but some of the California parks may be closed when you are planning to travel. Some of the California parks also seem to be eliminating their hiker/biker sites so it may help to travel with others through some sections. There are also many county parks along the coast so if there is not a state park check county web sites. I stayed in two county parks that were good. Both had hiker/biker sections.