It remains to be seen whether supporting regulations are a good enough proxy to avoid ring-fencing. The EU has now officially scrapped the proposals of the BSR bill. 

I guess they have a valid point in the fact that a number of "parallel" regulations from BCBS 239 to MIFID II to Volcker to FTRB is slowly but surely putting the checks and balances required to protect the average retail customer. 

However, it's only a matter of a few years to see what works (and what doesn't) as the 'Big Four' in the UK go ahead with their ring-fencing. And I'm sure once they figured out Brexit (or not), the regulators on each side will have ample time to exchange notes on the effectiveness of their respective regimes.