i am an irish O2 costumer and from this evening at 7 i will be in the uk until monday wondering will my phone work over there and what is the cost rate and internet rate for the UK.
This information might be of interest to you.
If you opt-in to the O2 Travel addon, and use the O2 UK network while roaming, calls are free to receive, otherwise its 9c per minute. Also outgoing calls are cheaper too, 25c per minute, instead of 30c with standard roaming: http://www.o2online.ie/o2/roaming/o2-travel/
How to get O2 Travel:
You have to opt-in to the O2 Travel addon, and be on the O2 UK network, to get the special O2 Travel rates. That might mean manually selecting O2 UK network on your phone.
Enjoy your trip.