They are definitely NOT carrots! Did you buy the seed from a company? I would get back to them and complain. It looks like some kind of weed but not one I recognise. It could be foreign if the seed was originally form abroad ( seed is not always sourced from UK).

Looks like a weed to me too... perhaps the carrot seed didn't germinate and this weed simply grew?

I've had a hunt through the cress family and thought of penny cress. But, what about garlic penny cress? Does it smell of garlic?

As a weed is a plant that shouldn't be there then by definition this is indeed a weed.

If your soil was contaminated with weeds, I would have thought that at least a few carrots would have come through as well.

It appears to me that the seeds you bought and planted as carrots were of something else and probably a mix-up in the packaging process. I would contact the seed company and email them the pictures. I'm sure that they will look into this as this could be bad for business and they will probably compensate you well in the form of lots of packets of seeds ;o)

Just had another thought. Maybe your seeds have somehow developed into a new strain of carrot? Now there's a thought, this time next year you could be a millionaire :o)