I had contact with the support of XXsim today. They told me, that my UK Number is already assigned and it is a regular UK number. My number is +44 7700 3XXXXX.
Researching it here
http://www.area-codes.org.uk/mobile/#077 I got told, it is indeed a "regular" mobile number, but from SURE from Jersey. Localphone charges me 19.9 Euro Cent per minute to call.
On the other hand, included minutes from mobile contracts do not diversify by carrier...
BK
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Mobiles: iPhone 7+, iPhone 7, iPhone SE, Blackberry Priv, Honor 8, Lumia 950
Prepaid-SIM: 3 UK, Save Money Sim, Toggle Mobile UK, XX-Sim, Koko Mobile