Hey Andy,
I think the best thing for you to do is to come over to Canada, get married, and then apply outland, with your wife sponsoring you.
You will send all completed application forms to Mississauga where they will assess your wife as a suitable sponsor, then the application will be forwarded to London.
I made a spreadsheet showing processing times through London VO and right now, average time from sending application to Mississauga to receiving PPR is about 106 days.
You should be aware though that it takes alot longer to gather all the documents together than you might think. You should download and print the checklist at least now, so you can start the process of collecting 'proof'. You also need a police clearance.
As you are from UK, you wont need a visitor visa, but you will need a return ticket. In the past as I entered Canada, I was never asked to show proof of my return, however, I had a one way ticket once and air transat wouldnt even let me check in until I had a return ticket. It cost a fortune cos I had to buy it at the airport. You should buy one even if you dont intend to use it, but dont tell the immigration officer that when you arrive at the airport!
If you have any other questions I'd be happy to help, you can also post on the outland London thread where there are lots of helpful people.
Good luck to you, and congrats on your engagement