my suggestion is much like the federal police stations on the US version of missionchief, but just on the UK version.
i'm suggesting another couple of units, which are country-wide services/units within the UK, like federal law enforcement in the US.
the US version has the federal police station, which provides them with FBI units, i think that there should be a building like this in the UK version that gives us NCA (national crime agency) units, which would handle organised crime like they do in real life.
the second unit would come under the same building (or not, it doesnt really matter), but it would be ctsfo, also known as counter terrorist specialist firearms officers, which would deal with the more terrorism-like calls.
these units and buildings could also come with other missions for them, such as 'bomb threat at railway station', 'bomb threat at airport' or 'mass shooting', etc.