Which Freqs work best in built up areas

[CharlesHarris]

UHF such as PMR446 or 70cm ham band work best in and around steel reinforced concrete and glass high rise buildings. A 500mw UHF portable like PMR446 will communicate vertically in an office building about 5 floors, or 5 city blocks horizontally at street level. If you are on an upper floor close to an unshielded window you may communicate up to 10 blocks or so if you have clear line of sight. But be aware that most windows have metallic film or mesh shielding to eliminate UV and to hinder eavesdropping.

In the US some office buildings have cell phone boosters to improve signal strength with portable phones and public safety radios. I don't know if these are common in UK.