I got some underpants from BHS, they are the type that have an opening in the front so you can whip it out and pee. But these sort of have a pouch, and my packer fits perfectly in it and it is perfect. It is like they were made for this purpose (obviously they weren't, but it's great). Also, they are really comfortable, which is always a plus. From what I've seen packing underwear seems to cost like £25+, but these were 3 for £15. Idk, I just figured some other guys might find this useful because it is a cheap alternative. Though I guess this is only great if you are in Britain because BHS is a British chain store...

What's the exact type (maybe the type name on the packet?)

The packet says "Atlantic Bay, Underwear, Trunks, Cotton Mix" that's all it says on the front. I don't think that's particularly helpful.

If they're the same type as the ones on the BHS website, then the design is called 'keyhole trunks'. I know they're also available at M&S and Peacocks in the UK, and at the very least on amazon.com for those in the US.

I've always thought that they might be good for packing, but since I don't currently have a packer I've never been able to test it. Good to know!

Thank you Jake, you made this a worthwhile thing. Thank you for finding the words I could not:) I'm not very good with words (as this has just proved)

Just wanted to add in case it helps anyone that I buy my boxer briefs from new look, which are great for packing as long you order a size smaller than you normally would for clothes.