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'

Full details of 'large letter' maximum :

Max length: 35.3cm
Max width: 25cm
Max thickness: 2.5cm
Max weight: 750g

If your 'package' is bigger than any of the above dimensions, it then becomes a Royal Mail 'parcel'. Then you will find Royal Mail is still around the cheapest but there's no need to consider it alone. For example myhermes may be 10p cheaper and has other factors that make it a preferred option.