Good thing the laws in Britain allow you to keep it
Yes in England the rules are very fair. It really belongs to the landowner but if anything valuable is found the value is split 50:50 with the finder. If two or more silver or gold coins are found together then they must be declared as treasure and handed in to the coroner. Eventually you will get a payment that a panel decides it is worth, which is always surprisingly low!
Any other object containing any gold or silver more than 300 years old must be handed in as well. In Scotland all coins have to be given up.
I have about 20+ silver coins now ranging from William The Lion 1180 to present day.