It is legal for you to dispose of the goods as long as you do everything 'by the book'. That means that you must provide reasonable notice (e.g. 28 days) in writing and that, if the letter is sent by post, you must use recorded delivery. The letter must clearly state the date on, or after, which you intend to dispose of the goods.
Here's a draft letter for you. (I'm assuming that you'll put your address at the top. That's not just courtesy; it's a legal requirement to include your address in the letter):
Dear Fred Bloggs:
TAKE NOTICE that I hereby impose an obligation upon you (under Part 1, Schedule 1of the Torts (Interference with Goods) Act 1977) to collect all items owned by you which are currently stored at the address stated above.
Further, TAKE NOTICE that, if such goods are not collected from this address by 20th January 2009, I will (on or after 21st January 2009) dispose of such goods, by sale or otherwise, and you will lose all title to such goods and any benefit from the sale thereof (in accordance with the provisions of Part 2, Schedule 1of the Torts (Interference with Goods) Act 1977).
Yours faithfully,
. . . . . . . . . . .
Chris