All praise is due to Allah. Salat and Salam be upon the Final Prophet Muhammad (Sallallahu alaihi wasallam), his relatives and the Sahaba.
Witr prayer is a very important Sunnah prayer. Prophet (Sallallahu alaihi wasallam) has urged the Sahaba to pray Witr prayer in many Ahadhith. This can be prayed in maximum 11 Raka'ats and minimum 1 Raka'at. However, the minimum complete number is 3 Raka'ats.
Different Narrations have mentioned different ways in which the Prophet (Sallallahu alaihi wasallam) performed Witr prayer. Based on that, Religious Scholars have arrived at the following three methods:
- Praying the first 2 Raka'ats together and the last 1 separately. This is the best method according to the view of many Scholars, including Imam Shafie.
- Praying 3 Raka'ats together and reciting 'Thashahhud' (Aththahiyyat) only in the last Raka'at and completing with Salam.
- Like in Maghrib Salah, praying 3 Raka'ats together and reciting 'Thashahhud' (Aththahiyyat) in the second and third Raka'ats, and finally giving Salam.
With some evidences, Hanafi Scholars say it is obligatory to pray according to the third method; but Scholars of the Shafie Mathhab say it is Makrooh (not preferable) to pray in this method, as the Prophet (Sallalla hu alaihi wasallam) has said not to pray Witr like Maghrib prayer.
It has been mentioned in your query that the followers of Hanafi Mathhab, after Taraweeh prayer, seperate themselves from the congregation for Witr prayer. This could be avoided, if the Imam (in spite of being a follower of the Shafie Mathhab) conducts Witr prayer according to the third method mentioned above. (Although it is not the preferred method, it is permissible within the Shafie Mathhab). In this manner, the followers of the Hanafi Mathhab will not have to leave the congregation, and will be able to pray behind the same Imam. Hence, the unity and harmony amongst the Muslims of London can be maintained.
Please note as usual it is Sunnah to recite Qunooth in the last Raka'at of Witr during second half of Ramadhan.
Allah knows the best.
Wassalamu alaikum warahmathullahi wabarakatuh.