To dial a UK mobile from the UK, just dial the mobile number as you normally would, whether the mobile user is at home or roaming internationally.
To dial a UK mobile number from a UK mobile that is roaming internationally, the most reliable method is to dial the number in full international format: replace the trunk prefix 0 with country code +44 (including the plus sign).
Alternatively, in some areas you may (or may not) be able to dial the UK mobile number in domestic format; however, doing so may also connect you to a number in the country where you're roaming, if the number ranges overlap.
To call a UK mobile from a UK landline, you always just dial the UK mobile number, exactly the same way you do when it is in the UK. The network will find the mobile automatically, and the mobile user will pay any applicable surcharges for international roaming.
Just as you would call an English landline.
+34 (including the plus symbol) followed by the 9-digit Spanish number
Where the UK mobile number is 07xxx xxxxxx dial either +44 7xxx xxxxxx or 00 44 7xxx xxxxxx. It doesn't matter which country the other phone is being used in.
Dial the UK mobile number in full international format: replace the trunk prefix 0 with telephone country code +44 (including the plus symbol).
England is the country name. People from England are English. Things from England are English. The language of England is English.
+44 is the international code for England.
Vodaphone
The physical location of a mobile phone is not relevant to how you dial it. To call a US-based mobile phone from the UK, dial 00 1 and the area code and number.
To call a UK-based mobile phone, anywhere in the world that it may be physically located, just dial the UK mobile numberexactly the same way you do when the mobile is in the UK.
English
English