If you are moving to a new area and want to check which network gives best coverage there, it is possible to travel to the area, do a manual search for networks on your mobile, and compare the relative strengths of the signals detected.
Whilst this is true, note that the home network of the SIM card is always set at the top of the list if in range, and only the rest are independently ordered by signal strength
Therefore a more realistic result comes either from using a foreign SIM with no preferred network affiliation, or using two or more UK SIMs and doing a bit of figuring in your head.
Note that sometimes a network has a stronger 3g signal than 2g, like T-mobile does where I live, and that sometimes the results are inconsistent anyway, due to signal strength fluctuating, and the transmitter power gets wound down in non-busy periods, though usually seems to pick up for this search.