Sorry thought you meant at see!!
No officially you are not permitted to fly it!
"Civilian use is permitted on land, but non-naval/military use at sea is prohibited. Unauthorised use of the flag in the 17th Century to avoid paying harbour duties – a privilege restricted to naval ships – caused James' successor, Charles I, to order that use of the flag on naval vessels be restricted to His Majesty's ships "upon pain of Our high displeasure." It remains a criminal offence under the Merchant Shipping (Registration, etc.) "
Union Jack - Wikipedia, the free encyclopedia