Twilio: $0.02/min to all US and $0.03 to UK landlines or $0.32 to UK mobiles -- yes, 16x the US rate.
Voxeo: $0.02/min to all US and UK landlines or between $0.14 and $0.35 for mobiles.
Google wouldn't use Twilio or Voxeo for this kind of service so it gets cheaper elsewhere -- especially on their scale -- but it shows the relative difference.
UK rates really are a crapshoot. Voxeo and Twilio must just be averaging the rates for mobiles. The rates I get at Flowroute for UK mobiles vary wildly: https://www.flowroute.com/services/rates/U/
Twilio: $0.02/min to all US and $0.03 to UK landlines or $0.32 to UK mobiles -- yes, 16x the US rate.
Voxeo: $0.02/min to all US and UK landlines or between $0.14 and $0.35 for mobiles.
Google wouldn't use Twilio or Voxeo for this kind of service so it gets cheaper elsewhere -- especially on their scale -- but it shows the relative difference.