You dial the same as if the mobile phone was still in the UK.
So, if you are calling from the UK you dial the full mobile number 07xxx xxxxxx.
If you are calling from outside the UK, you dial:
+44 7xxx xxxxxx
(The plus sign means "insert your international access prefix here." From a GSM mobile phone, you can enter the number in full international format, starting with the plus sign. The most common prefix is 00, but North America (USA, Canada, etc.) uses 011, Japan uses 010, Australia uses 0011, and many other countries use different prefixes.)
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.
+44 (for the UK), then the full mobile number without the leading zero. for example, to call 07999 999999, you would dial 0044 7999 999999
To call Spain from any GSM mobile, anywhere in the world, dial +34 and then the 9-digit Spanish number (landline or mobile).
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.
+34 (including the plus symbol) followed by the 9-digit Spanish number
Customers can call 789 from their Virgin Mobile phone to call Virgin Mobile UK. Other customers can call 0845 650 4500.
To call a UK-based mobile phone, wherever in the world it may be physically located, you always just dial the UK mobile number the same way you do when the mobile is in the UK. The network will automatically locate the mobile and re-route the call accordingly, and the mobile user will pay any applicable surcharges for roaming.
You can't.
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.
If you're calling a UK mobile that is physically in Spain, just dial the UK mobile number. It does not matter where the mobile is physically located. If you're calling a Spanish mobile, dial 00 (the prefix for outbound international calls from the UK), then 34 (the country code for Spain), and then the 9-digit Spanish mobile number. If you're calling from a GSM mobile phone, you can dial +34 (including the plus symbol) instead of 00 34.
The fact that the UK mobile is in France has no bearing on the cost to the original caller. The caller will pay the normal charge for a call to a UK mobile number, and the mobile user will pay any applicable surcharge for the international roaming.
Dial the UK mobile number in full international format: replace the trunk prefix 0 with telephone country code +44 (including the plus symbol).