You should check in London with the Embassy or High Commission of your home country, as they may stipulate that your Deed Poll must be witnessed by a solicitor or Notary Public, for it to be effective in your home country. If your Deed Poll has to be witnessed by a solicitor or Notary Public, they will charge between £5 and £15 to do so, and you should check which documents they also need to see. It is likely that you will need to take your birth certificate and/or passport with you when you visit the solicitor or Notary Public, so please check with them before you go to meet them.
Full instructions on execution (signing, dating and witnessing) of your Deed Poll will be sent to you with your Deed. For more information, please go to the Deed Poll Information list at the top left of this page and click on Signing/Witnessing/Execution.