United MileagePlus (Consolidated) - interline with IcelandAir

I am flying UA SFO-LHR and then connecting on a separate ticket to
Reykjavik (KEF) on IcelandAir. Will UA check my bag to KEF at SFO?
I have 7 hours between flights at LHR. Similarly on the return, will
IcelandAir check my bag to SFO connecting via LHR? I only have 2h25m
at LHR so time will be of the essence. I have a US passport.

One of my relatives had no problem with an interline between US Airways and Icelandair- the bag was checked both ways all the way through, and Icelandair (after many requests) even waived the baggage fees since the US Airways ticket was in first class.

I know US is not UA, but hopefully it'll work the same way.