As you indicate, it's not simple! That's because (by splitting a consignment into several packages) you could not only be reducing weights but also sizes. I'll try to summarise it for you. (Where I've given two prices they're for 1st Class and 2nd Class respectively)
A parcel is 'small' if the dimensions are no bigger than 45cm x 35cm x 8cm OR 35cm x 25cm x 16cm. If it weighs less than 1kg it costs £4.10/£3.70. If it weighs more, but under the maximum weight of 2kg, you pay £7.95/£6.70.
A parcel is 'medium' if it's bigger than above but less than 61cm x 46cm x 46cm. Up to 1kg costs £5.65/£5.20. Up to 2kg, £8.90/£8.00. Up to 5kg £15.10/£13.35 (or £12.92 by ParcelForce's 48-hour service). Up to 10kg, £21.25/£19.65 (or £15.92 by Parcelforce). Up to 20kg, £32.40/£27.70 (or £27.68 by Parcelforce).
Larger parcels have to go by ParcelForce (at the rates in brackets above).
So, for example, you might have a 'medium' parcel that weighs just under 2kg, which will cost you £8 to send Second Class. If you split it into 2 parcels, each weighing just under 1kg, it will cost you MORE (£10.40) if they're still 'medium' but LESS (£7.40) if they're both now 'small. However you might spend more than 60p extra in packaging anyway, so you'd not actually save anything.
http://www.royalmail.com/sites/default/files/RM_Our_Prices_OCt2013.pdf