Hi there!
This is my first post and you have no idea how pleased I am to have found this discussion board.
I'm in the same situation. I am canadian, I met my boyfriend in Peru while doing volunteer work and after finishing my courses a year later, I applied for a working holiday maker visa which (after the rules changed 4 times while over) expired and I had to return home. I had a job where my superiors really appreciated my work and were sad to see me go. They applied for a work permit in April 2005 (which failed). So I decided to go back to university in Canada for autumn 2005 ended up staying in the UK on "holiday" from may 2005 until september when my post grad courses started.
We have decided to get married this coming summer to facilitate immigration. Therefore, I've done quite a bit of (obsessive) research on the matter. In regards to your particular situation, I recommend that you get married in Canada and apply from there. It seem you need to apply from the country you are resident. Beside that point, there is the matter of processing times for the application and costs.
Applying from within the UK for a change to your current status (visitor to spouse) seems to take ages according to the Home Office website in comparison to the High Commission in Ottawa (15 working days although some on this board seem to claim that it takes much less, comments?)
Also, I had looked into getting entry clearance as a "fiancé" but, you end up paying more than double since you have to apply initially for the fiancé visa then again for change to your current status from within the UK. Considering plane tickets already cost a fortune, I opted for the less costly option and do the application from Canada.
I don't think you can "send" your passport the High Comission in Ottawa because you need to get in the UK with that visa (so you need your entry stamp on it which I suspect needs to be relatively close to the date you mentionned in your initial application.) And the High Comission state on their website that they treat applications only for people who are in Canada and mention that if you are in the UK, you need to contact the Home Office instead.
Let me know how it goes! I'm doing the same thing next summer. So much timing involved...
Right, for a first post, I think it was too long
Cheers!
Gen