sylvday didn't read the entire thread. The answer to the question is no, its not. The envelopes are reply envelopes; the sender will pay the post fee for the envelope to be returned to them. To divert it elsewhere means there is no agreement to pay the fee, thus its the equivalent of posting an envelope without a stamp. If it gets delivered you've defrauded the post office, or they may ask the recipient to pay postage.
Surely using them as a "regular envelope" implies adding postage, though? I don't know for sure, but would suspect that as you've been sent the envelope, then it's yours to use as you see fit subject to legal restrictions on you (e.g. filling it with cocaine = illegal).
Why not rip up the junk mail into little bits, put it into the pre-paid envelope and send it back to the return address. Not only are you letting companies know how annoyed with junk mail you are, but you are also costing them the price of the postage to receive your junk. It certainly cheers me up after a particularly bad day.
With regard to Pinotage's comment on my answer, I did specify that I didn't know if this practice was legal or not. That said, my understanding is that postage on these envelopes HAS been pre-paid, whether or not they are used - see the Royal Mail web-page http://www.royalmail.com/portal/rm/jump2?mediaId=6 00027&catId=400043
I thought all the envelopes came with a small box with the Business Reply Service Licence Number in? Surely this means that the envelopes are processed by the Royal Mail, and then mail costs are charged to the company accordingly? Also, most charity envelopes have a pre-paid reply envelope, and they usually say that using a stamp will save them having to pay for costs? Just my two cents!
You're righton righto. Reply envelopes' postage is only charged to the company concerned when it goes through the postal service. Putting a stamp over the 'reply paid' means a charity will not be charged the postage when it goes through the system. Anything that comes through your letterbox (or left on your doorstep) is not legally yours as of right. I like the tearing up pamphlet and sending it back in the envelope suggestion, sweet justice!
I do the same as skelly, and I have now progressed to adding other junk stuff to the envelope as well. A friend of mine advocates filling a shoe box with stones or anything else, taping the envelope to the box and sending it. I have an old bed I want rid of. I wonder.....hhmmmm?