Question regarding Joining Maghrib & Isha during the Summer time in the UK

Qu 8 – The time for the Isha Salah in the UK, in the Summer time comes in very late; is it permissible for those who are young or are in need to gather between Maghrib and Isha (i.e. join Maghrib & Isha), if yes when do they pray the Witr prayer?

[Q&A with  Shaykh Ubayd bin Abdullāh Al-Jābirī, April 2009, Madinah]