Did you advise your creditors that you were at a different address out of the country?
You need to look at what the legal status of such a demand and whetehr the sender needs proof you received it or whether it's enough just to post it
http://www.insolvency.gov.uk/pdfs/forms/form6- 1.pdf
Look at the Civil procedure Rules. Here's an extract:
Under the Civil Procedure Rules and the Insolvency Rules 1986, "substituted service" is acceptable as long as the conditions that would permit "substituted service" of the BR petition itself are met.
"PRACTICE DIRECTION � INSOLVENCY PROCEEDINGS
11.1 The creditor is under an obligation to do all that is reasonable to bring the statutory demand to the debtor's attention and, if practicable, to cause personal service to be effected. Where it is not possible to effect prompt personal service, service may be effected by other means such as first class post or by insertion through a letter box.