2.65c/min on Plivo? The other prefixes are more 'premium rate' numbers, not mobile numbers. That's about 1.5p/min.
Considering Plivo charges 1.2c/min for US numbers as a benchmark, the difference between US and UK numbers is 1.45c/min or... 0.88p/min, exactly the wholesale cost pretty much. So I think it is working.
It might be that wholesale rates are capped but it clearly isn't pushing prices down for consumers who want to call into mobile numbers.