When sending demos ensure you make the label aware that you are available for remix work! Trust me on this one 
I guess when u release more than 4 tracks in a labell they will send u remix parts and stuff. but one question i have. how working on remixes will help u in your producer career ?
Skream basically exploded his career on the back of remix.
If you come up with a corker it can make a massive difference, especially as often the original is a more established artist, so can sometimes add some credibility, or if they are a chart act (like in the Skream/La Roux case) it can get radio play and more.
cool. i might start working on remixes soon.
I have done alot of remixes / edits of tunes and got them signed (and out soon on big labels)
if you can do a good remix the label will really pay attention… if its really good they will clear copyright and release it like i have had
It shows what you can do with parts and they will probably give you more … it does help to show full originals too
check my soundcloud you will see what i mean other than the cr2 records track which isnt up there until release 
My tip of the day:
You touch it, you wash it.
don’t eat yellow snow