London Heathrow to Euston

1) It depence on the services, some services would pick up at Central Bus Station only while some would pull into each individual terminal~~ So if you are to take the National Express, then be sure when you book the services is actually a services that operate via Terminal 5~~

2) Again depence on the services that you take~~ But in general majority would pick up at both Terminal 5 and Central Bus Station, but some would only pick up at Central Bus Station~~ When you book the ticket, make sure you select Heathrow Terminal 5 then you should be alright~~

3) It really depence on the traffic, as BA is a morning flight which means you will need to go through the rush hours traffic~~ Usually the time taken from Heathrow Airport to Central London is some 45minutes, but in rush hours this could actually double~~ And the cost should be GBP4 per trip if they hasn't change the price~~

Personally, I would suggest you to take the Underground whihch is much easier and away from the traffic~~ Just follow the sign to "Underground" at Terminal 5~~ The Piccadilly Line will bring you directly into Central London~~ For Euston, you will need to change at South Kensington~~ Change form the West/North Bound Central Line (via Kings Cross St> Pancras) and get off at Euston Square which is just outside Wuston Railway Station~~ The gtrip would cost you GBP4 for one way, or GBP3.2 for Oyster Card user~~

The most convenience way you better take the underground or Heathrow Express.
Undergournd - one day pass is cheaper if you travel more in same day. It costs STG 5.60.
Heathorw Express costs STG over STG 10.00