I have never been denied CCR access when arriving to LHR in F and departing to KBP the next day. I always explain the issue to the agent at the door to CCR and ask him/her to check with the agents inside if we can come in and we are always invited to come in. Many West Coast services into LHR don't connect to a lot of shorthaul destinations. While other OW lounges may not be accessible due to OW rules being stricter I see no problem with the OP having access to GF.