Better to think in terms of a currency rather than a pair.
The notion of 'over' or 'under' value in the crosses then can often apply in the short term.
For example, right now markets are just open and thin - Eur/Gbp Eur/Usd and Gbp/Usd
First has opened down, second is up and third is stale.
So whilst I type I expect Eur/Gbp to rise, based on the buying on Eur/Usd and the lack of Gbp action on Gbp/Usd.
Will it continue, it's suggesting Euro buying, so depends on USD, then check tomorrow morning to see.
Eur/Gbp 88.77, Eur/Usd:2013 Gbp/Usd : 3500 - remember thin markets,.