Another member of the brassica family from which so many of our familiar vegetables derive, things like broccoli, kale, turnips and swedes even things like bok choi.
younger leaves clasp round the stem as can be seen here, and they have a bluey green colouration but it's not as distinct as that of Brassica napus ( oilseed rape) and is not as emerald green as that of charlock.
as the plant grows it produces the standard yellow crucifer type flowers, and the ID can be difficult between the various species, subspecies and cultivars of Brassica rapa, though there are a few tips to look out for..
The bluey green colour is one give away but so is the fact that the buds often grow slightly higher than the flowers!!! though obviously you'll need to see the plant in flower to use this trick, but the last hint is one of the best....
.. if you check the basal leaves and it has lots of little raised warts or spots then that's generally a dead give away for wild turnip.
Although due to the fact that there are so many subspecies of this genus I realise identification can be difficult and using just one of the tips above can give you a good idea of what you've found though you might still not be 100% sure as the tips are not always fool proof, however find a plant that fulfils all the criteria above and you've found Wild Turnip.