Depending where in, "continental Europe," you are flying to will depend where your flight from LHR departs from. While most of those destinations (and I believe most if not all long-haul BA flights) are from T5, BA does have a few departing from T3. When I flew to TIA last month, the BA flight was out of T3. Although I arrived on BA at T5, I don't think there was any option to go into the terminal proper, just get on the train, bus to T3, then security over there. It really wasn't a horrible process at all, from flight to lounge in maybe 30-35 minutes (I had just missed a bus so there was a short wait for the next).
This thread in the BA forum is kept fairly up to date with the BA flights out of T3:
List of BA flights from T3
Also, I believe BA F is also included in the $200 AARP discount (like business). Plus you can get the 10% discount if you have the BA Chase card (you don't need to pay with it); all segments must be on BA metal to get the Chase discount. Yes, you can stack those discounts.