OK, having done this with one 20month old, here is what I advise.
Ring the airline & find out what luggage allowance your twins are allowed. We were told none by the travel agent but the airline said different (was years ago now, so I can't recall exactly what dd got) & to check if they get meals. Again, travel agent told us no, airline said yes. Apparently TA's tell you that children aren't entitled to anything as a way to get you to pay for it, or but a seat!
Put away some toys now, so they are 'new' on the plane.
DO NOT take anything with lots of parts, the last thing you need is to be crawling on the floor looking for bits. Dd had a bead frame, a colouring pad & some playdough. But she LOVED in the entertainment & rarely took off the earphones!
They do supply some things on board, but what dd was given was unsuitable!
Pack changes of clothes for all of you in hand luggage. Dh pooh poohed this & didn't pack anything in his HL, he was soon glad that I had a spare pair of everything when dd spilt milk all over him within in a hour of take off!
Keep nappies/wipes etc in one small toilet bag. learn to put nappies on in confined spaces
Do not bank on them sleeping much! I know that it seems they will, because they should...but it doesn't work that way! Dd was so excited that she barely slept. We managed to get her a bassinet, because she was still tiny for her age, but as soon as we hit turbulence, we had to life her out, which seemed to happen each time we got her settled! Again, you could try the airline & see if you can get a bassinet, but they may be too heavy.
Is it just the three of you travelling? Or will you have help? If you have help, one of you eat while the other is feeding/helping the twins. Then the other eats after. You cannot eat & help the twins at the same time! If they don't get meals, ask the flight attendant to tell you an hour in advance of meals so you can get the children fed. Saying that dd didn't eat much, mainly snacked, so be prepared for that!
You will be called first to board as you have children.
Stay on the plan until last, then you don't have to worry about all the pushing & shoving while trying to get the twins off.
Make sure you have buggies for them. We had a stay over of 3 hours & dd slept most of that (boring airport surrounds)
Get them to drink while you take off/land to help with ear pressure.
I think you would be daft to take seats/porta cots. Ask your family to hire them here, before you arrive. Our were hired at a dollar a day & yes, it is expensive in the long run, but cheaper than bringing them on flight! The car seat situation, they aren't stricter here, they just have a different tethering system. They use an anchor strap from the back of the seat that clips onto an anchor in the car. These cannot be fitted to UK seats.
Try to get to the airport early to get bulkhead seats (as long as there aren't too many families needing bassinets). The extra room means that the twins can sit on the floor.
With it being term time, there maybe some unoccupied seats available on the flight, the flight attendants usually as people to shift about a bit & try to free up the seats near you, which would be great! They did this for us on the way home, so dd got a seat!
Don't count on getting much sleep for a couple of day when you get here. It was very hard to stop dd falling asleep at daft times for hours on end for the first couple of days!
Do not worry about other passengers! It is a small amount of time in their lives & you are just as entitled to fly as they are, as are your children!
Have fun!