The answer to this is simple. Find the correct department at Virgin that deals with hardware collections. Write them a letter.
State that after repeated attempt to get them to collect their gear it is still with you. Since the contract is terminated, and they have acknowledged this in their correspondence (date & reference info) you are making your final attempt to get them to fulfill their obligation and collect the equipment. If they fail to do so "within a reasonable amount of time" (and that's actually a legal phrase, so it carries some weight, and say 60 days from date of post mark) then they will forfeit any claim to the equipment and you will dispose of it at your leisure.
Make two copies of the letter. Send one to Virgin by recorded delivery. This last bit is important. You want a name and date for when the letter is received at their end. If they claim not to have received it you have proof. If they lose it internally, or ignore it or do nothing then that's their problem not yours. Make sure you include your full contact details along with any Virgin contract number so there's no excuse that they didn't know what the letter was referring to. Keep the second copy and attach the recorded delivery receipt you get from the post office. Allow 5 days for delivery. Check the RM web site and get the tracking & delivery confirmation. Print it out if possible. Attach to your copy of the letter.
This might sound a little OTT but you are ensuring that there's no legal comeback on you should you decide to go back to Virgin in the future.