I
> notice that the Tesco brand organic eggs are from roaming chickens.

doesnt guarantee that their range support  insects and greens  for them (
most likelly not if a big scale farmer )

> Does anyone know if they are also grain fed?

yes are grains seems necessarry to insure egg production in a commercial
operation ( via hormonal disturbance )

 I would not be suprised
> if the "standard" organic eggs had better omega-3 levels than the
> Columbus ones, and probably have better overall nutrional value too.

organic eggs are not necessarry better if coming from a big scale producer.
what makes a real omega 3 rich eggs is the diet rich in greens and  smalls
animals .this can be achieve only when the chickens are raised in a very
small farm where they can roam free or they are in a pen moved daily  on big
pastures .

flax seed fed chickens migt have more omega 3 s but i doubt the egg is as
well balanced that coming from a really free ranged chickens .( as the main
ingredients of the feed is wheat and soya and other omega 6 rich grains )
by the way legally the label free range means only that each chickens have
something like 4 square feets of range ( not enough to feed it )

jean-claude