In general whatever you find on your detecting day with us will be yours to keep and take home with you. However there are some exceptions to this.
If you are lucky enough to dig up any object made of silver or gold that is three hundred years old or more, or it provides an exceptional insight into an aspect of national or regional history, archaeology or culture, then by the law it must be declared through the Treasure Process under the terms of the amended Treasure Act 1996. In practice this means the object must be reported to the local Find Liaison Officer, and possibly deposited with them. A decision will then be made on whether or not it classifies as "Treasure". If it is, then the State will claim it and financial compensation representing the value of the object will be given to the landowner and finder (to be split 55/45) If not the object is returned.
The same applies to any 'hoard' that is found. And a hoard can be defined as little as two or more coins found together.
The convention in metal detecting is that any item of value that is found should be split with the landowner. On Discover Detecting an 'item of value' is defined as anything with a value judged to be more than £500. In the event of this occurring, the item could be sold on the open market and the proceeds split between the finder and the landowner, or one or the other party could agree to 'buy out' the other for half of its value, that sum being gauged by a qualified valuer. Discover Detecting will retain 10% of the value of any such item as compensation for the cost and time of processing the documentation and arranging valuations etc.