As long as the UK is an EU member, they have the right to be represented in the EP. From an EU-27 perspective, this might be treated as a UK problem, but these these British are still EU citizens and would have good a good case in court.
As long as the UK is an EU member, they have the right to vote (& stand!) in the EP elections. Should legally not be a problem, but might slightly affect the result in areas with many British citizens. The least problematic bit.
If the UK would fail to organise EP elections and the EP thus had to convence without British MEPs, all decisions this 'EP27' would take could then be questioned in court. This would cast a huge shadow over EU decisions in critical times.
b) an extension until the end of 2019 at the least, to secure citizens rights, the EU institutional transition and give the UK time to find a different political path either by general elections or a 2nd referendum or both. /ends