Can I check in for UA over here?

In October I am flying to IAD on VS and thereon to GSO on United. My connecting flight on the same PNR is two hours after we are scheduled to land in IAD.
Some years ago Aer John and I were booked to JAX from LHR via IAD on VS/UA. Our onward flight was at 17:30 - long after we had landed in IAD. We cleared Immigration and Customs and in the baggage hall our bags were collected for onward transfer and we were directed to the UA desk therein to check in for our flight to JAX. When we got there I was told there was a problem and directed to the gate. At the gate I was told there was only one seat available and a boarding pass for me was printed. None for Aer John who was 14 at the time!! [:0] The gate agent told me he was #7 in line after the elites and then it went to #10 and I freaked! At this stage it was 16:00 and we had been on the road since 23:00 the previous day their time. He tried to route us though other airlines but we were being met in JAX and I had no way of contacting and in any case at that stage I didn't really want to land in Jax at midnight having been re-routed via CLT or CVG or any other hub you care to mention. I wanted my direct flight. The gate agent was good. I'll give you that but he kept on pressing whatever button he had to show me that there was no availability (and other elites were coming forward demanding seats) when all of a sudden the machine went "ping" and out popped a boarding card for my baby!!! I clutched it like a drowning man clutches a straw and although they had assigned him an exit row I kept schtum grateful to be on the flight. Let them sort it out on board (They didn't!)
I'm anxious to avoid a similar occurrence this time. OK, it matters less than the last time. We were going to a wedding; there were two of us; mobile/cell phones weren't as prevalent and we didn't have tri-band. I can manage a delay this time if I have to, but I don't want to if I can help it. Therefore I am asking whether, having checked in with VS and had my bags tagged all the way through (even though I have to customs clear them at IAD) can I check in on the IAD-GSO route at LHR?
Some years ago Aer John and I were booked to JAX from LHR via IAD on VS/UA. Our onward flight was at 17:30 - long after we had landed in IAD. We cleared Immigration and Customs and in the baggage hall our bags were collected for onward transfer and we were directed to the UA desk therein to check in for our flight to JAX. When we got there I was told there was a problem and directed to the gate. At the gate I was told there was only one seat available and a boarding pass for me was printed. None for Aer John who was 14 at the time!! [:0] The gate agent told me he was #7 in line after the elites and then it went to #10 and I freaked! At this stage it was 16:00 and we had been on the road since 23:00 the previous day their time. He tried to route us though other airlines but we were being met in JAX and I had no way of contacting and in any case at that stage I didn't really want to land in Jax at midnight having been re-routed via CLT or CVG or any other hub you care to mention. I wanted my direct flight. The gate agent was good. I'll give you that but he kept on pressing whatever button he had to show me that there was no availability (and other elites were coming forward demanding seats) when all of a sudden the machine went "ping" and out popped a boarding card for my baby!!! I clutched it like a drowning man clutches a straw and although they had assigned him an exit row I kept schtum grateful to be on the flight. Let them sort it out on board (They didn't!)
I'm anxious to avoid a similar occurrence this time. OK, it matters less than the last time. We were going to a wedding; there were two of us; mobile/cell phones weren't as prevalent and we didn't have tri-band. I can manage a delay this time if I have to, but I don't want to if I can help it. Therefore I am asking whether, having checked in with VS and had my bags tagged all the way through (even though I have to customs clear them at IAD) can I check in on the IAD-GSO route at LHR?