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Declining an ITa doesn't ruin future chances.

I am thinking your ITA should still be valid since your LMIA doesn't say you should be in Canada. This is a guess tho.
 
If I just went for it, I need to change my addresses to UK, country of residence, and under working in Canada select no and then cannot upload details of my LMIA anymore. If i wrote a detailed letter of explanation that everything is the same as it was at the time of ita, just I am not in Canada anymore, and a scan of my work permit, I guess it will still go through the 6 month process time aswell. Its not like I can get a quick answer and then go for a new LMIA.. So this is where my predicament is. My head is battered I'm almost tempted to just decline and come back when I'm 100% certain of my application. Anyone reading this that wants to put their 2 cents in please feel free!
 
Can your employer write you a letter that you are on an unpaid leave? Because that is what this all sounds to me. If you get that letter, you can attach your letter of explanation that you are in the UK for that leave. You will then put Canada as country of residence and UK as correspondence address.

Since you left in April, you don't need PCC for UK nor change of address.