Is that related to the specific phone, or to the Tesco system?
There is no Tesco system as such, at least not as far as the basic operational side goes. Tesco just takes care of the billing, issuing of SIM cards, etc. All the physical network operations are contracted out.
There are only five companies providing the actual networks in the U.K. Hutchison is the Johnny-come-lately of the bunch and is only providing 3G services. The four GSM900/1800 cellular networks are Vodafone, O2, T-Mobile (U.K.), and Orange (O2 is what was originally BT Cellnet and T-Mobile was formerly One-2-One, part of Mercury Communications).
Only Vodafone and O2 operate on both the 900 and 1800MHz bands. T-Mobile and Orange are 1800MHz only, and coverage in more rural areas is often less comprehensive (as visitors to this area using these networks often discover). This also means that tri-band phones from the U.S. which don't have the 1800MHz band can only interwork on the Vodafone and O2 networks here.