I'm always up for a sample swap - get in touch via Twitter.
If you don't have any empty sample bottles a great place for them is https://www.ampulla.co.uk, I normally get these ones with these caps.
I get my labels from a great little business on ebay here, but you can just write on sticky labels if you want!
I try to use ex-Master of Malt boxes but anything sturdy will do just remember to wrap all the bottles well in bubble wrap!
You can normally post within the UK for £3 using 2nd class Small Parcel.
Now the Royal Mail will only allow you to send two bottles through the post, they can be up to 70cl but only 2.... Not useful when you want to send 5 samples. Some Post Offices ask you what's in the parcel and how many bottles then refuse to take it.
Your best bet is to use Click & Drop where you buy the postage online, print your label, stick it to the parcel and either drop it off at a post office or if you can find one put it in a parcel box or post box with a wide opening. No awkward questions! You even get 10p off the price if you use this method.
DM me for my current open list.