Sadly, it's not actually a freepost address according to their website.
On the other hand, they do have a freephone number - 0800 587 6 587 - so you could ring 'em, and leave the phone off the hook while you're at work.
( ,
Fri 2 May 2014, 8:33,
archived)
Or put the lowest denomination stamp you can buy on the envelope
then they have to pay the shortfall and £1.00 administration fee to receive the 'correspondence'.
( ,
Fri 2 May 2014, 8:56,
archived)
:D
I love the reaction of the woman in the background
youtu.be/Znq4H_1F_bY?t=12s
( ,
Fri 2 May 2014, 9:49,
archived)
youtu.be/Znq4H_1F_bY?t=12s