You're estimating 4380 kWh a year production which is not realistic. There's no doubt a 2.4 kWp system could produce 12 kWh a day on a good summers day, but as anyone that lives in the UK knows, "a good summers day" are few and far between. In Winter it could very well make next to nothing on many days. Just to back that up, the energy savings trust say a 2.5 kWp system here in Cambridge, on a south facing roof, with no shade at a 30 degree angle (I.E. perfect/optimal conditions) it would produce 2337 kWh annually. Near half what you estimate.
Then you have to factor in if you can even use all the energy you produce. Highly unlikely without a battery because most households aren't drawing ~2.4 kW for a large portion of the day, so you'll be exporting (for a small gain), then buying energy back in the evenings at high cost. the EST estimate true saving of only £400 a year, assuming your at home all the time (which is again unlikely for most!).
Unfortunately, anyone slapping 8 panels on the roof hoping to save £1500/yr really will be in for bill shock! Solar is a great option, but you really do have to keep the numbers realistic!