im fairly sure you are not supposed to but people do and assuming you arent upsetting anyone who is really going to know or care.
'Just call yourself a traveller'
It is against one of the various Town & Country Planning Acts to do so.
You can probably get away with it for a month but after that, expect a visit from a Planning Officer. Unless no-one notices and/or the local authority do not get to find out.
One would have to be there for in excess of 4 years in order to establish a permanent right for the caravan to be occupied there.