http://www.uk.castingcallpro.com/
http://www.talentcircle.org/
http://www.starnow.co.uk/
http://www.tobeseen.co.uk/
Posting an ad on either of the four above (Perhaps Gumtree, aswell) will go a long way to finding the actors you need. Ofcourse, it's favourable that you pay your actors, whether that's minimal, or the standard fee. However, many are willing to work for expenses/eats purely for experience. There are thousands of Actors that
want to be involved in as many projects as possible, and would be willing to be apart of your production.
Be sure that the locations are set, that the dates/script are finalised, and that you have the necessary crew to get the very best out of your material before you post an ad. It's a very important factor when not paying your talent. They have no obligations to leaving you high and dry if you reshedule and it's not suitable for themselves, so be sure to tick the boxes before you cast.
The same can be said for finding your crew. It's not necessary in such an early stage to go out and purchase the very best equipment. Give yourself a chance. Advertise for local film-makers, or University students that are building their CV and looking to get involved. Find collaborators with their own equipment. There are, again, thousands of film-makers willing to collaborate. Networking is a huge part.
Remember, collaborating is a "Give and take" deal. Be sure to let them know your help is always there. That you'll pass the names of the cast, and crew, to any other friends in the business you have.
Best of luck.