I posted them a few days back, and people have started receiving them. Only so far about half of my guests have had to pay extra to collect my invites! I'm highly annoyed by this, as I checked the weight (waaay below the cut-off for standard 1st class letters) and in the Post Office checked a few of them through the little letter box slot thing to check they weren't too thick (they weren't, really, just a little thicker than a normal letter). The half-dozen overseas letters didn't incur extra cost either.
So far only a couple of people have actually had to pay (6p plus £1 handling charge) but four people have had stickers on their invites to say 'extra postage to pay' but the letter was delivered anyway! I'm quite annoyed by this situation because the first official point of contact people have with my wedding is now slightly sullied by the extra postage charge. I also have a few lazy cousins and other relatives who probably wouldn't bother to collect a letter that they had to pay a charge for, so I'm having to let them know about this via email.
So I guess a word of warning for UK brides - Check, check and double-check postage costs for invites that might be even slightly thicker of heavier than usual! Don't fall into this trap!