Losing a deed poll is more common than you might think, and the solution is simpler than most people expect. If you obtained your deed poll from an online service like this one, you can usually download the original PDF again from the service. If you cannot access the original file, you can create a new deed poll with the same details, sign and witness it again, and it carries the same legal weight as the original. There is no deed poll registry; a deed poll is only as authoritative as your signature on it, so a freshly signed document is fully valid.
An unenrolled deed poll (the type this service generates) is a private document. Unlike an enrolled deed poll, there is no public record of it, which means there is no central authority to contact for a replacement. The practical approach is to create and sign a new one. The new deed poll will have today's date rather than the original date, but this does not affect its legal validity. Organisations care that the document is correctly formatted and properly signed, not the specific date it was created.
If you have already used your original deed poll to update some records (your passport, driving licence, bank account), those updates remain in place. You only need the replacement deed poll to update any remaining organisations. In the meantime, if an organisation asks for your deed poll and you cannot provide it, explain that you are in the process of obtaining a replacement. Most will allow you to return once you have the new document rather than turn you away entirely.