I think I'd probably go for the trusted friend option too, as if there's anything important, at least they can open it for you. Failing that, however...
I haven't used the service, but looked into it earlier this year - Royal Mail will hold your mail for up to 2 months at a time - I know this isn't long enough for a 6 month trip - but maybe you could arrange for 3 lots of 2 months in advance at the local sorting office/post office...
Royal Mail Keepsafe ServiceThey'll also forward your mail onto another Post Office with the Post Restante service - but you need to know where you're going to be so you can pick it up!! (which can be a challenge if you're just packing and going wherever the mood/wind takes you...)
It could be long shot, but you could combine the two services, and after 2 months get everything sent post restante, and then repeat the process till you're ready to come home (although you may not have much ability to deal with the paperwork from abroad/you may not want to carry it around with you if it gets sent to you... that or you send it all back to yourself...and half the point of getting away is to get away from all that post!).
Hope this helps - have a great time.