How is a traffic pattern oriented, entered and exited? If you've received traffic entry instructions from Heathrow Tower (sim) to fly left downwind for runway 9L, does that mean to be to the left of runway 9L (for a right-turning traffic pattern, where downwind is north of the runway) or to enter a left-turning traffic pattern? I ask because the latter would take you over runway 9R/27L and I'm unsure whether this is a problem or whether it's ok because of the difference in altitude between a plane in 9L's downwind and 9R's/27L's final/upwind.

Also, if you've received clearance from runway 9L to depart north, should the departure be done from the traffic pattern? E.g. you take-off, turn crosswind, turn downwind then turn base and depart the airspace? Or should you continue straight until you've left the airspace, then change course?
The instruction to join a "left downwind" is used to clear an aircraft into a standard traffic pattern with lefthand turns. The word "left" could be omitted, as only right turns and righthand traffic patterns are called with "right". In the case of EGLL - London Heathrow the standard pattern would be a northern traffic pattern for runways 09L/09R and a southern traffic pattern for runway 27L/27R.

If you should come to the unlikely situation that you are VFR and want to depart from Heathrow, the clearance to leave the CTR in a northerly direction would be best executed leaving the crosswind, but clarification on frequency never killed anybody:

A: Heathrow Tower, G-ABCD, request leave CTR direct northbound after crosswind 09L.


or

A: Heathrow Tower, G-ABCD, request heading 360 after turning crosswind 09L.


When departing an aerodrome with traffic, it is unwise to leave the CTR along the extended centerline/upwind, as you are impeding other traffic, especially in slower aircraft.