Hi,
Are you having call drop outs or data loss.
Try turing off 3g and use 2g which would mean slower speeds.
Remember 02 are improving their masts and will be sharing with Vodaphone and with the soon to be release of 4G fully by 2015, hopefully the service will improve over time.
TuGo may be an option.
http://www.o2.co.uk/tugo/want-it
Go into settings, general, reset, reset network settings.
Think about swapping your sim card just incase.
You can pick one up from any 02 shop and do the swao instore or at home.
http://swapmysim.o2.co.uk/
Did you check if 02 gave you good coverage by trying out one of their pay sims first before leaving Virgin.
Try your sim in another handset to see if signal improves or not.
If the checker states the masts are working ok but your unsure then report the mast to 02 by ringing 202 for contract customers or 4445 for payg.
their are booster boxes but they are for business customers only.
You may have to think about changing networks if your not locked into a contract.
Apple will run a diagnostic on the phone if you feel its at fault.
http://www.apple.com/uk/retail/geniusbar/
