The first lesson should be about intoducing yourself. A PPT with pictures of your family and activities. That should last a week, as you probably only see each class once a week.  I don't teach elementary, I teach middle school and that is how my schedule is.

As mentioned above, the first week or so will most likely be your introduction and observations. Luckily, I had about a week of observation then the second week I started to co-teach starting with the activities/games.

Any holiday themed items will be good for later on in the year, definitely!-e.g. maybe a Christmas hat, Halloween masks or even just decor.  If you'll be working in an elementary school then you could probably use them for English Festivals or Camp~

Hope that helps.
Good luck with packing!

I made my introduction so that it was interactive. It worked well with my students. For example, instead of just showing pictures of my family, I put a fill in the blank statement with multiple choice answers. Ex: This is my ____________ A) brother B) sister C) pet.
If you make the answers more obvious and add some funny answers then they tend to stay more engaged.

 If you like something from Korea then let them know. It is kind of a bonding thing.

  It is also good to sometimes give them hints for questions to ask if you have a free question period. I tend to pretend I am measuring how tall I am or looking at my shoe size.
If there is something surprising about you then make sure to include that. I write my name on the board in Korean at the beginning. I will also have a translation period where I will say some basic Korean and they will translate into English. I teach middle school though so this may be a bit advanced for elementary.
I would definitely introduce your reward system if you have one and any rules that you may have.
I have the students make name tags with information about them. It gives me a chance to say hi to individual students and learn more about them. It also gives me my first glimpse of what level of English they are at.
I play a simple game with them at the end if there is time left over.

If you have any halloween stamps or stickers then it would be a nice reward during the month of October.
I just visited home and brought a lot of small reward gifts back. I'm not sure about the U.K but in Canada we have some awesome yet cheap English stickers. Shiny ones are popular.
My best advice is to keep it simple, show no fear, be approachable, and love your students.

i had to start teaching the very first day i arrived at my school. make sure you have some activities prepared besides the whole intro ppt and asking questions thing. for my first week i did an intro ppt and asked the students if they had any questions, and very, very few of them had questions. so i was left with over 20 minutes in class with nothing to do the very first day. talk about awkward. had to pull something out of the hat in class at the last minute.

so make sure you do something like 2 truths and 1 lie, maybe even a simple quiz on your home country that the students can play, etc.

i think there are some "introductions" lessons in the lesson plans section if you have a look there, probably under "non-textbook based lessons"

Seems all that could be said has been said. I would just like to reiterate an important point.

Teach your rules!
Develop 5 rules for your class, with tier consequences of about 3.
Type them out and print a copy for each student, pass those out in class.
Go over your rules and procedures in class.

I don't bother with introduction lessons, its silly and a waste of time. Your students have had hundreds of foreigners before you, you are not new or neat to them. And unless you are a famous Pop star you are not interesting. Just a simple, "I'm Mrs.Swimmer, your new English teacher!". The focus should remain on your students and their education.

Instead jump straight into a lesson like a real teacher. Your first lesson should be an assessment, you need to know what your students know. So test and assess their speaking ability, as you are now their speaking coach.

You can get a basic idea from the Korean teachers surrounding your student's levels, but do not rely on their assessment as they do not focus on student's speaking ability.

Pinpoint gaps, and take lots of notes, this will help you design your future lessons. You can also ask students what they know, what they don't know, and what they want to learn.  This keeps the class focused on them.

I was also thinking of bringing some Christmas / halloween items over with me to base some lessons around.

They know what Christmas and Halloween is, and you can find Christmas and Halloween type stuff here in Korea if need be.