Someone left a long comment below a random post on this blog about how Royal Mail deals with freepost and pre-paid envelopes. The person has apparently spoken with Royal Mail's customer service department about whether or not they would return, say, a freepost envelope stuck to a brick (kudos if true). Allegedly, our anonymous investigator was told the following…

• Items should be within the standard weight limit of 20kg (that's enough to return a couple of bricks).
• However, sticking, say, an A6-sized freepost envelop to a shoebox containing your bricks shouldn't work – apparently the size and weight should be roughly the same as the original freepost / pre-paid envelope.
• Items that are rejected are returned to the sender, so an easy way around this is to write the junk mailer's address on the back (so that it will still be delivered to them).

As said, I'm not sure if all this is true. I might also add that the rules for freepost addresses (rather than freepost envelopes) are presumably more relaxed – on the basis of what would Royal Mail not deliver a box containing a brick to a freepost address?

In any case, thanks to whoever shared the information!