data usage on roaming network?

[Terra]

I was going to get a new smartphone with Virgin on pay as you go where I top it up with £15 per month and I get 1GB of data. Where I live it's sometimes hard to get a Virgin signal and goes onto an Orange roaming network. If I was to say use facebook or something when on a roaming network, would it start depleting my phone credit or use up my data allowance more quickly or does it have no effect at all? Also in general if I run out of my data allowance do I need to top up my phone with £15 to be able to use internet apps again or can I carry on using them and it just uses up my phone credit instead and then I put another £15 when the credit runs out? Thanks.