The shortest distance between two lines being a great circle, I'd look towards Ukraine for the likely explanation; given that HMG referred The Guardian to the T

The shortest distance between two lines being a great circle, I'd look towards Ukraine for the likely explanation; given that HMG referred The Guardian to the Treasury, RBS probably was made aware that there are sanctions either on RT or people or organizations connected to it. Keep an eye on the Consolidated List of Targets (UK's OFAC List, essentially) for additions over the next week or so -- that's likely to be the source of the rationale. (Addition: Also, if Assange is using the embassy's network access, then I would be dubious of any claims that it's been "cut off," as the UK FO isn't going to willfully violate Vienna Convention strictures over Wikileaks, at least not without a stiffly worded demarche and a whole lot of legal cover.)