From what can be gathered from the posts at TUAW, O2 POS (Point of Sales, not Piece of S***) software systems only take the last 9 digits (with the first 10 or 11 locked by software). T-Mobile and Orange are unknown.
If Vodafone's system allowed an override of the first 11 digits to allow you to specify a complete 20-digit replacement USIM number for your mobile number (that's how you can have your SIM replaced and keep your number), you enter the full AT&T USIM number and it accepts it, you can associate it with your actual UK phone number.
After all, the subscriber details are dependent on the SIM card, its (U)SIM number's association with a mobile number in the provider's HLR (Home Location Register), and an account associated with both. If two of the three items stay the same, and your POS allows you to replace the third at will in its completeness without verifying the country code, then you should be golden.
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Not necessarily a red herring
From what can be gathered from the posts at TUAW, O2 POS (Point of Sales, not Piece of S***) software systems only take the last 9 digits (with the first 10 or 11 locked by software). T-Mobile and Orange are unknown.
If Vodafone's system allowed an override of the first 11 digits to allow you to specify a complete 20-digit replacement USIM number for your mobile number (that's how you can have your SIM replaced and keep your number), you enter the full AT&T USIM number and it accepts it, you can associate it with your actual UK phone number.
After all, the subscriber details are dependent on the SIM card, its (U)SIM number's association with a mobile number in the provider's HLR (Home Location Register), and an account associated with both. If two of the three items stay the same, and your POS allows you to replace the third at will in its completeness without verifying the country code, then you should be golden.