I have been there about 5 years ago (DEC, worked there for 3 years), so it might be a bit different today.
First day was group exercises (2) followed by an interview. Interview was competency based, so you might want to prepare for that.
Second day was SIM. For DEC you get an easyJet FO. He will follow orders but not show any initiative.
For me it was some raw data flying with raw data ILS. I dont really remember details, but nothing spectacular.
Second part was a LOFT style exercise. In my case departure from Gattwick, some minor malfunction in the climb out.
Then a hydraulic leak with winter weather at destination. I decided to return to Gattwick.
Bur other guys had different scenarios...
Some advice: They love communication. Make a thorough DODAR/FORDEC and a good approach briefing.
If you have several options, choose the safest option and they will be happy
Third part was a online personality test, followed by an interiew about the test.
Good luck..