Area code creating apple ID
UK numbers are all 11 digits, or country code plus 10 digits, omitting the leading zero.

so either
01234 567 890
or
+44 1234 567 890
[spaces only for legibility].  

UK mobiles all start with 07 [or +44 7] - so the 'area code' is 07 nnn or +44 7 nnn

Just for extra information, adding your own country code to all domestic numbers does not affect call cost whilst in that territory, these days the systems are smart enough to figure it out.

00 international designations are only for dialling from landlines which can't use the + symbol.