.....I don't use cruise control in built up areas because it is dangerous and irresponsible. Yes you can set a maximum speed if you have that option....
Not correct on both counts.
Cruise control is not useful in stop-start conditions or where the traffic flow is slow. It is useful in flowing traffic and it is safe to use in those conditions. It is a bit easier in an automatic, although most cruise controls these days let you change gear without cancelling it. It is safe because if you touch the brake it is immediately switched off.
If you need to accelerate a bit you can and then the cruise control kicks in again when you take your foot off the throttle and the speed has reduced. Sometimes you have to brake back down to "normal" speed but then you simple re-set the cruise control instead of letting the engine braking do it. Some cruise controls will even use the brakes to maintain speed. If you don't have the brake function then stay in a lower gear.
Most cars will still use less or the same amount of fuel in 3rd gear at 30mph than in 4th, that's because the road isn't flat, you're always going up and down slight inclines and 3rd gear is more efficient because it is gives better engine braking (using no fuel) down the slopey bits and better torque, therefore more efficient use of fuel when going up the slopey bits.
As for using a speed limiter, that is a no-no in a built up area, despite what Foggy says about there being no reason to exceed the speed limit, a prod on the throttle that takes you briefly over the speed limit may well be required. For example an unforeseen situation where someone like the tw@t Kryten was telling us about, suddenly does something stupid and you need to pour on a bit of coal to do an evasive maneuver.
Anyone who says that they are an observant, careful and defensive driver is also telling you that they have the capability to be flexible enough to quickly evaluate and react to a situation that occurs which their observation skills cannot predict.