When is Royal Mail no longer cheapest ?

When does Royal Mail stop being the cheapest to send a package ?

Answer :

If it's bigger than Royal Mail 'large letter'

Then it becomes a 'parcel' (probably 'small parcel')

For parcels :
Myhermes may be 10p cheaper
 
When does Royal Mail 'large letter' become a 'parcel' ?

Answer :

if  bigger than 2.5cm deep     (I point this out as it's the main disqualifier for my packages)

see below for full 'maximums'