As BA6501 says, LHR-BEY flights are in very nice lie-flat beds. The BA lounge in T1 is fine and in some ways better than the club lounge in T5, especially when the latter is crowded. If you were to fly BA to IST you'd be in a standard CE seat by comparison to CW. Having said that, Istanbul is a magnificent city and if you have time would be well worth visiting on your way to Beirut. There are plenty of flights from IST to BEY on either Turkish or MEA, the excellent Lebanese airline, and it's just over an hour.