Hello AMCP!
Ok, I'll try my best to give my opinion.
I've used those 2nd Class UK stamps before, and this is what happend:
I wrote to Cristiano Ronaldo with a small
4x6 sized SASE with
one 2nd Class UK stamp and I got it back with no problems.
I also wrote to Daniel Craig and Sir Ian McKellen with
8x6 sized SASE's with also
one 2nd Class UK stamp. I got Sir Ian's autograph back in my SASE, but he added a 68p stamp.
I also got Daniel's autograph back, but he used my SASE and didn't add any extra postage. It seems it was delayed because of that as I got a sticker on the back of the envelope saying so... :neutral:
So, my opinion is that you should at least put 3 2nd Class UK stamps on an 8x10 envelope (sorry, I'm South Africa, we don't work with ouces here so I hope it's ok if rather I talk inches!
).
Good luck and if you have any more questions, please feel free to ask!
Regards,
Lizanne