In North Europe in summer Isha prayer is at midnight. In the mosques, Isha prayer follows Maghrib prayers immediately. The logic behind that is understood, but my question concerns praying at home, can we do the sames, i.e. praying Isha after Maghrib, even though we will stay awake after that watching television or doing any other thing till the proper Isha time and some times well beyond it?

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

The time of 'Isha (night prayer) begins (without any disagreement) when the red twilight disappears.
If the disappearance of red twilight in a place delays till midnight then the inhabitants of that place should perform every prayer on its fixed time. They are not allowed to perform 'Isha prayer with Maghrib by reason of the delay in the disappearance of red twilight or the shortness of the night, since these are not legal excuses.
Therefore, all of you should perform every prayer in its fixed time. Also, the male Muslims should perform the 'Isha prayer in congregation and you should exhort those who are combining two prayers without any legal reason.
Finally, we warn against watching forbidden matters on TV, since it leads to many evils. For more details, please read the Fatwa: 84333.
Allah knows best.