It would be very nice if you could just do a network scan in a location and find out the signal strength for all networks. As far as I know (on Android anyway), you can only find out what networks are available, if you want to know the signal you must use a sim card and connect to the network. Any free sim card on PAYG will do the job.
Thinking about it, when I use my Samsung without a sim card, it connects to a network by it's self and shows emergency calls only. It doesn't actually say which network, but when you check the MNC in an app like Network Signal Info, you can see which network it is. MNCs are as follows:
O2 = 10
Voda = 15
Three = 20
EE = 30
Orange = 33
I guess it picks the network with the best signal, the last few times I've booted my phone without a sim it's used O2 and Three. But when you go into network selection and try to choose a network, it doesn't allow you to register, so you don't really get a choice as to which network your phone hangs onto, unless there's some way to force that.