I will try this a second time. For some peculiar reason the original did not post correctly.
It is not just the UK discussing the revocation of citizenship of those who have joined Isis. In fact it appears a popular procedure.

Australia has revoked the citizenship of five people who traveled to Iraq and Syria to join Islamic State, bringing the total number of people stripped of their citizenship to six.
Changes to the Australian Citizenship Act passed that year(216) automatically revoke a person’s citizenship if they engage in terrorism-related conduct, including those who are in the service of a declared terrorist organization overseas.

Danish government reaches agreement to revoke passports of Isis fighters
The new rules will provide for an administrative process that will enable passports to be revoked without going through the courts, the Ministry of Immigration and Integration announced.

Switzerland wants to strip ISIS fighter of passport.The Swiss justice minister on Monday confirmed that the country was looking at revoking the passport of a Swiss citizen who fought for the so-called Islamic State (ISIS).

       Germany already strips anyone of citizenship who joins a foreign army. Since 1999, a German who joins the armed forces “or similar armed organisation” of another state without the permission of the German defence ministry forfeits their German passport.

       France has been embroiled in a similar debate. Three years ago, François Hollande, the previous president, abandoned a constitutional bill that would have allowed dual nationals convicted of terrorist acts to be stripped of French citizenship after he failed to win sufficient support in parliament. But rightwing politicians have continued to demand such a step.

And to add some balance it must be quoted:
A day after Corbyn said ISIS bride should be let back into UK...Labour's John McDonnell once called for anyone who fought for Israel to be stripped of their British citizenship

    John McDonnell wrote to Theresa May in 2014 about Britons fighting for Israel
    He said: 'Will you be warning any British citizens considering engagement with the IDF that such engagement may put their British citizenship in jeopardy?'
    Yet Labour leader says ISIS bride Shamima Begum has 'right to return' to Britain   !!!!!!!