The country code for the UK is 44. In front of 44 you need to dial the international access code applicable to where you are. In most European countries it's 00. In the USA and Canada it 011. GSM cellphones (which are clever!) will automatically select the appropriate international access code if you dial +.
So:
0870 translates into
0044 870 from Europe
01144 870 from the USA and Canada
+44 870 from GSM cellphones worldwide