Smell and stalk and leaves are the giveaways. If it smells like parsley, and the stem has a kind of thick U shape to it where it joins the root, and the leaves have a gentle wave to them, then look up flat leaved parsley. It's hardier here than the curly stuff.
That said, those are big and dark for this time of year, and BR is spot on about the blasted buttercups.
(That's a generic blasted buttercup incidentally; they're all blasted buttercups 
)
http://en.wikipedia.org/wiki/Parsley
This is where I'm happy to be proved wrong since we all learn; but afaik there are no toxic plants that smell of parsley, that aren't. Cow Parsley apart, there are a couple that
look like flat parsley, but none smell like it. Queen Anne's lace smells of carrots, while hemlock water dropwort has hollow U shaped stems, smells of celery and has a cluster of roots that are the colour of parsnips (called dead man's fingers), and with this mild, wet Winter, it's already up and showing fresh green leaves.
http://jpwaldron.wordpress.com/poisonous-greens-a-z/
Cow parsley looks like this,
http://www.woodland-ways.co.uk/blog/wp-content/uploads/2013/01/cow-parsley.jpg
http://www.eatweeds.co.uk/sweet-pickled-cow-parsley-stems
Sometimes English is confusing, especially when searching the internet; the familiar Queen Anne's lace is the name give to both wild carrot and cow parsley, and in North America it's different again, but those posts are widely used on the internet.
Old allotment, left to run....if it doesn't smell of parsley, it's blasted buttercups
M
p.s. Looks like a nice set up there
shaded in parts but it has potential
SWMBO was having a dig around and thinks she's come across some parsley however i'm no forager, anyone care to shed some light?
