I have been on O2 for the last 3 years since i moved here and it has always been rubbish indoors. I recently upgraded to a 4G iPhone on O2 and it does seem to be a lot better.
My son has also had O2 which was rubbish and has just transferred to a 3 sim card, and he is now finally able to take calls indoors (which is great because now our landline isn't ringing all the time while he organises his social life!)
I think what works where really does depend on your location in this area.
I did a bit of informal research by just taking note of when people were able to make and receive calls in my house and what network and phone they were using.
We are on Virgin broadband and I know they do some very good phone deals - but I do know that they used to be a bit limited as they only had a small slice of the bandwidth - that may have changed.
For what it's worth, my sister uses (and has bought all her phones at) Tesco Mobile, as do her husband and kids and it works very well - and she gets a signal in my house!