Possessory Tile (based on adverse possession) is a 'holding' title. No-one can register deeds-based ownership unless the Chief Land Registrar first serves Notice on you* and considers that the possessory title was wrongly awarded. Yes, you* can build on it or sell it; but, in either case, an Indemnity Insurance policy is essential.
If no deeds-based ownership challenge or registration is lodged within twelve years from the date of first registration, you* can apply for the title to be upgraded (fee currently £40) to absolute; and thereafter no-one can disturb it at all.
* -i.e. you or your then successor
JEFFREY SHAW, solicitor [and Topic Expert], Nether Edge Law*
1. Public advice is believed accurate, but I accept no legal responsibility except to direct-paying private clients.
2. Telephone advice: see http://www.landlordzone.co.uk/forums/showthread.php?t=34638.
3. For paid advice about conveyancing/leaseholds/L&T, contact me* and become a private client.
4. *- Contact info: click on my name (blue-highlight link).