Hi, welcome to the board and congrats on your babies! I am living in Aberdeen, Scotland with my DH and 11 month old DD. I look forward to reading your intro when you get a chance-there are a couple of us now in Scotland :)
In regards to your questions, I will try to help! Having two to feed at once though sounds a lot more challenging than one-if you have not tried already-you might want to ask the multiples board this question as well.
In case you delayed in any way and if you need the pump to be able to feed both twins, I would bring it as a carry on. What type of pump is it manual/electic? (keeping in mind the different electric in mainland Europe as well as UK) Also-medela makes sanitizing/cleaning wipes that you can quickly and easily wipe down pump parts when on the go. If you do not normally have to pump to be able to feed both babies, then I would consider checking the pump in your luggage if you already have a lot to carry on-which I imagine you do with twins!
Also, in case you run into an emergency and something happens to your milk, I would consider bringing formula along.
I would rather bring the chilled milk as opposed to frozen (I think chilled lasts longer) and would bring enough to last 24 hours of feeding one baby. The flight attendents can be very accomodating-they can bring a cup of hot water for you to heat the milk and some will even heat it up for you, if you have your hands full.
When we travelled when DD was little I would pack the following in my diaper bag for the flight: enough diapers for changing every three hours for the duaration of the trip (including layovers) plus a couple extra, wipes, a couple pacifiers with the clip to attach to their outfits (the only time DD ever used one was on a plane-when she would not feed during takeoff/landing I would give her a pacifcier to suck), two extra outfits (I found the footed sleep n plays the easiest for travelling), a large ziploc bag to store the soiled outfit in case of a diaper blowout, changing pad, and 1 or 2 clip on toys (your babies are so young though they probably wont need them)
I do not want to overwhelm you with info! If you have any other questions, I would be happy to help. The other ladies on this board are very knowledgeable about travelling with babies and will offer you advice too.
Also, try not to stress too much (I have a tendency to do that) Yes, the trip will most likely be long, tiring, and stressful at times-but it is only 1 day of travelling-you can do it :) ...Scotland is also an amazing part of the UK, we absolutely love living here!!