Which UK banks currently offer Open Banking services?

All financial institutions that currently offer an online payment account in the UK and Europe are obliged to provide Open Banking interfaces. In the UK, the nine largest banks and building societies were responsible for defining and providing these interfaces as part of the UK's Open Banking initiative. These banks are known as the 'Competition and Markets Authority 9' or more commonly, the 'CMA9'. These are: 

AIB Group UK (trading as First Trust Bank in Northern Ireland); 
Bank of Ireland (UK); Barclays Bank; 

HSBC Group (including First Direct and M&S); 

Lloyds Banking Group (including Bank of Scotland and Halifax); 

Nationwide Building Society; 

Northern Bank Limited (trading as Danske Bank); 

The Royal Bank of Scotland Group (including NatWest and Ulster Bank); 

and, 

Santander UK. 

Aside from these, many other financial institutions can and do choose to take part in Open Banking.