This is probably covered by the torts Interference with goods act 1977. After 3 years it would not be unreasonable to consider the goods to be abandoned, you have a duty of care and you may be held liable for any damage while they are in your care.
If you wish to act strictly within the law make a reasonable effort to find the address of the owner, if impossible write to the last known address and say you intend to dispose of the goods and give them a telephone number to contact you to arrange collection within, say, the next month (a month is a reasonable time and English law likes reasonableness). If you hear nothing dispose of the goods but remember if you sell the goods the money is not yours, retain it for return to the owner for the next 6 years, do not make any charge for the storage and you should keep a copy of your letter and your attempts to find the address of the owner and any other transactions.