You will not get anywhwere near the same number of channels on Freeview as you do on Sky+.
You wont of course get any of the Sky channels, plus you wont get the channels on Sky you have to pay for normally (non Sky channels that are part of the Sky package).
Why not just stop paying Sky any money and then just pick up all the "free" channels on the Sky platform, I believe there are over 200 of them.
Note you wont have to pay ANY money to Sky if you already have the Sky dish and Sky box. Any money mentioned on the web site below is for people who DONT have a Sky dish or box. There is NO monthly charge.
Note if you do that you lose the recording facilities on the Sky+ box, so you can still watch then channels, just not record them
More here
http://www.sky.com/shop/freesat/home/