We went with O2 because we were spending time in the UK and in Europe and they promised EU roaming At the time I reviewed them, their reviews were all good unfortunately. They have since been taken over by Virgin Media. Our experience was very poor and most upsetting. We ended up having to dump the SIM card in Europe and buy a new one in Austria, basically doubling our cost. Their system also accepted a top up without providing the sevice and customer service was impossible to contact. Previous interaction with customer service was extremely variable from professional and kind and full of overpromises which were not delivered to rude and unhelpful at their store. Don't touch them with a barge pole if you intend to travel between the UK and Europe.