In my case, it was (still is) a prepaid number I used.
I made a call (to Germany, a cell phone) from a Belgian cell number in London.
I pay my normal fee (within Belgium) plus a supplement of (at that time) €0.27 a minute. No formal changes, nothing I had to do at all!
The only thing is: you pay a supplement for calling outside the UK. It's (rate dependent on either the exchange and your provider!) 26 €urocents per minute plus the local charges when calling either the UK, or the host country
On the other way around, you also pay for recieving (taken) calls!
Txt messages are normal rate, reception is free. That's roughly it.
(but I have 3 different numbers, one Belgian, as I live there, one Duth, as I had one before I emigrated, and one German. Cheaper for the Germans to call me, but more expensive for people at home..