Indeed! Why not separate London and all other areas in the UK that voted remain as well as NI? The point here is the Republic of Ireland have established a rule for their own country: to vote again on membership. If the UK leaves without a deal it could be very bad for The Republic. The Republic joining the UK would solve the border problem and to be quite frank I think the EU would prefer that. They wouldn't then take the blame for putting in a hard border.
There is another issue as well. The UK is one of the biggest contributors to the EU budget. When we leave assuming we leave on WTO we'll no longer contribute to the EU. So what will happen to some of those countries who receive more than they put in? Are Germany, France and Italy going to pay more to subsidise those countries who have got used to these entitlements? Are they going to pay the UKs share? I wouldn't be so sure judging by France's riots.