Orange and T-Mobile both have substandard indoor coverage compared to O2 and Vodafone due to them both being stuck on an 1800Mhz carrier frequency which lacks the pentetrative ability of the others with their 900Mhz carrier, so O2 and Voda are always going to win for coverage realistically.

When you look at the stats for coverage they are all outdoor readings, but indoor readings are often more relevent these days for mobiles.