You have no lounge access in LHR and DUB unless you pay for it.
Your UA status doesn't qualify you and your cabin of travel doesn't either.
Your departing flight from LHR is on EI (not part of *A) so you don't gain access.
Your only option is to pay for lounge access at EI lounge in T2 at LHR. When you check in with EI you will be given the option to purchase or you can pay at the lounge itself.
You can also purchase access to the lounge (51st & Green) at DUB which is after US pre-clearance.