US & UK taxes - File separately or jointly?

Foreign social security payments are what the UK government call "relevant foreign income". This kind of income is neither exempt under UK domestic law or the UK/US tax treaty, but could be excluded from UK tax for anyone who claims the remittance basis and does not remit that income to the UK.

ITEPA 2003 s681 says explicitly that foreign benefits which are comparable to non-taxable UK benefits are non taxable.

Sadly the interaction between the two tax systems leads to jargon words like "remittance basis" which are not intended to bamboozle, but demonstrate that these rules are open to interpretation.

 

How does the remittance/arising basis have any relevance here and how does it demonstrate that rules are open to interpretation?