I have to visit London without much planned ahead but did had option to continue my number site to international roaming charges. Some of my freind suggested me to try matrix as they provided sim card and number even before arriving in destination country. So I called up their customer care and found to be helpful. They explained me their plans and suggested me various options which I found to be helpful.
I decided before leaving a day earlier to call them and they were very quick and delivered sim to me at my home address the same day. I did not face any activation problem when I landed in uk and had good experiences with them. Thanks Matrix