If you're set on the garden select and you can't find any UK shops that sell it, you can find some ebay uk listings, that say they'll ship internationally. The main concerns I'd have with that, is the authenticity of the product, as well as freshness(not a bag over expiration). So you'd want to find a seller that seems reputable with good reviews. Also, there's likely to be a higher shipping fee, so you'd need to check that out to see if it's worth it to you.
If not, there are several UK shops that sell the regular oxbow adult and junior pellets, which I assume are the same pellet size as the garden select, though I don't know for sure.
Science selective is a very good UK brand, and I believe they're the smaller pellet size, though you should double check that to be sure.
Any pellet size can potentially be a choking hazard for a bun that wolfs down their food too quickly. If this is a significant concern of yours, you could try scatter feeding, as that can sometimes help slow a buns eating down. Or use something to break the pellets up into smaller pieces that won't be as likely to get stuck if not chewed well. You can also warm soak pellets to significantly minimize the choking hazard. Though this will depend on if your bun will eat pellets as a mush. And it does need to be made up fresh in a batch just large enough for one sitting. You just don't want excess left sitting out for too long, where bacteria or mold growth could occur.