Customs won't know if you took a bike over to the UK or not so no need to go to the trouble.
I bought a 09 Zesty 514 while I was in the UK last year. I rode it from the shop to where I was staying via a few parks to get some dirt on the tires. Luckily the bike box was a little short so the back tire was sticking out through a slot I had cut. Customs wanted to look at the bike to see it's condition but when the guy saw the dirty tire he didn't bother opening the box. Just don't get it too dirty as customs will want it properly cleaned before clearing it.
I hired the Spicy 516 and Zesty 714 for a few days at Glentress trail centre in Scotland,
both were awesome bikes, the 160mm travel spicy climbs like a mountain goat and descends like its on rails.
I bought the less expensive Zesty 514 on the same holiday, packed it in an old bike box and customs in Melbourne didn't even blink and eye.
Have yourself a holiday, drink some beer & ride home (once through customs)
I was over in the UK in June an was lucky enough to have 5 day riding the 7Stanes in scotland on carbon X-Control demo bike. The bike (and the trails) were fantastic, the shop i borrowed it from was looking to sell the bike on and I was sooo tempted to bring it back with me. But having just bought a new roadie here and the unknown of getting it through custom I didn't go ahead with it - and I'm still kicking myself I didn't
I also tried out a Zesty 714 which was awesome as well but it felt a bit too plush for me.
Hey guys I don't know if this of any help but a bike shop in my local Island of Guernsey back home (UK) deals with Lapierre. They recently posted a froggy to my friend who is currently in Queenstown NZ. Not sure who else they would post to but worth asking I guess.
These guys are pretty good value to deal with, it used to be my LBS before I moved here.