My post seems to come to CCR on a regular basis! (Including from those "in college" to use Harlem argot).

But yes, I'm regularly having to send out stuff printed off in the CCR printer (the toner is a bit low, by the way) waltz over to WH Smith to get stamps and envelopes. You can also buy postage online these days too. Then off to the post box by the middle escalators (after Plane Food). The entry slit on the box is very small, it won't take a package. For anyone querying this in the future, there are several postboxes in landside departures, at the back of the check-in counters, again letters and postcards only.

Your other option, which is super fantastic for the regular traveller constantly verging into admin chaos, is:

http://www.pc2paper.co.uk/mainpage.asp?USID=

You upload a PDF out of Word, Outlook or whatever, by 15:00 hrs UK time, they post it first class, second class, recorded, whatever the same day. I've used them for years and we can recommend them without hesitation.