Both the sponsor application and the PR application should be submitted at the same time to CPC Mississauga. Once a decision has been made (currently taking 70 days) they will forward the application to London.
London usually take another 3-4 months to make a decision, but it can be up to 8 months. When you receive the visa you will have until the expiry date of the medical to 'land', this will be the expiry date of the visa. (The medical expires 1 year after it took place, and it has to take place before you submit to CPC-M)
Once landed you can then return to the UK if necessary, you don't have to move right away.
In order to maintain PR status your husband will have to spend 2 out of 5 years living in Canada.
I don't know what your reasons are for not being able to submit the PR application yet, but like I said, you only need to 'land' before the visa expires, not move.
Other proof of returning to canada could be contact you have made with a real estate agent... or printouts of job searches in the area. Letters from family stating you will stay with them until you get on your feet. Or quotes from moving/shipping companies.
Join the outland London thread for more questions.
http://www.canadavisa.com/canada-immigration-discussion-board/thread-for-outland-london-uk-applications-t49436.2955.html