Thank you all for your replies. It was very helpful and I have enquired with Turkish Airlines.

However, they were very vague about my next questions so I'm hoping I can get a more detailed answer here.

Firstly, once checking in online and arriving 3 hours prior to the departure time, since we don't need to wait in line for the check in desk, where do we place our baggage? I'm aware we need to get baggage tags from the counter but dropping our baggage off, I'm not so sure. I am departing from Heathrow Terminal 3.

Secondly, since we are connecting at Turkey, Istanbul Ataturk, will we able to print out both boarding passes for both flights, for Heathrow to Istanbul and for Istanbul to our destination? Also, if yes, can we print out both boarding passes for our arrival back to Heathrow?

Thirdly, since we will have to print out our boarding papers at home, once we arrive at the airport, do we go straight from dropping our baggage to security checks and from there wait in the departure lounge for our flight? Is this what the 'Fast Track' signs at the airport mean?

Fourthly, on our reservation, we might have selected an aisle seat and window seat as our preference. Can I change my seat preference to any other seats for those that I have specified when checking in online? So, if I've chosen a window seat, can I change this seat to a non-aisle seat? Or will I only be allowed to choose window seats?

Fifthly, this 'liquids and baggage' allowance and rules are the most annoying. I'm travelling economy, internationally and the Turkish Airline website
says 20kg is allowed per adult. However, my family is convinced it's 30kg
per person...! Someone explain that it isn't, or am I wrong?

Finally, I have found out that one plane we are travelling in is a three by three seat plane, meaning there is only one aisle. This sounds quite crammed? Anyone experienced this type of seat plan before since I'm fairly new to this all new smaller plane than what I usually fly on.

Thank you all once again.