I went from Portugal to the UK for the interview and it was 100% worth it. The Assessment Day is a very positive experience in itself! The recruiters were all Cabin Managers based in London Gatwick (the base I applied for) or in London Luton and they were all really friendly and tried to make us feel comfortable and enjoy the day from the first moment. You meet a lot of people (I tried to talk with other candidates whenever I could, even though I wasn't able to meet half of them - we were about 50 in the morning) and group exercises and presentations are fun and relaxed - as much as an Assessment Day can be. We were divided in groups of 5 or 6 per table with two recruiters looking at us while we interacted with each other and taking notes.
After the lunch break (a lot of us ate at their canteen, that was lovely, but the airport was right next to it) we went back to the room we had been in the morning and they called about 25 names (I just know the room became half empty) and those were the people who left the room and didn't make it to the interview. The rest of us listened to another presentation (this time more focused on salaries, contracts, the training course, etc) and were given interview times (all of them in the beginning of the afternoon). I was on the first round and I was so happy that I had passed that I was pretty focused (and not stressing a lot like I imagined I'd be) on the interview and was able to have something good to say in all the questions. I was interviewed by two of the Cabin Managers that had been with us throughout the day and they were really friendly! The interview was around 35 minutes long but I felt like time flew, I was enjoying talking to them. At the end you can ask them questions (I asked things like what they enjoyed the most about working for easyJet or how it was like to work in such a huge base like Gatwick).
I left after the interview (it was about 2pm) and I got the job three days later. I'm now waiting for a start date :D