If you're using a mobile phone with a UK sim card in it, simply dialling the usual UK number (or calling it from memory) should work. That's because your call is automatically routed to your UK phone provider which (apart from the cost!) treats the call in exactly the same way as if you were in the UK. [It's always worked for me when I've made calls from abroad].
Otherwise you need to enter the international access code for the country which you're in. That varies but pressing + should do it automatically anyway. Then enter the UK code (44) followed by the UK number (omitting the initial '0').