If U.K. still an EU member after 31 Oct, it will have the legal duty to nominate a candidate Commissioner. If it does not,the Commission can continue to work and decide legally (precedent Bangemann 1999). The U.K. will be brought to the EUCJ for violation of its obligation
1/ In other words, the fact that HMG would violate the EU Treaties by not nominating a candidate Commissioner would not make the U.K. a non member State.
2/ To know if UK would still be an EU State if the PM violated applicable domestic/constit laws by not asking a prorogation of article 50 delay would not be an issue of EU law and for the EU Court of Just but for British competent authorities including perhaps its Supreme Court
