Hi Blair,
I'm a bit late responding as I've been away.
I have taken bikes to and from the UK on 3 occasions and the only time I've been charged XS baggage was on a Frequent Flyer award trip. All of these trips have been with QANTAS. (I have always been a little bit over the alloweance, but I think a KG or two doesn't phase them)
Twice I have brought new bikes home, the second time being just a few weeks ago. First time was in 2005 (Thorn Club Tour) and there was a queue at the VAT counter. (Go there before you check in) The VAT process took around 10 to 15 minutes.
This time I arrived about an hour early and there was no-one at the VAT counter. And.... I didn't even have to open the box. The attendant just asked me if the bike in the box was the same one that was on the invoice. Check in was easy enough but involved taking it to be weighed at the oversize luggage area as the scales at the check in have a bar across them that is too low for a bike box to fit under. I guess that's how they determine what is oversized?? Again, no one looked in the box even though I hadn't taped it up yet in anticipation of having to let someone have a look. While we were weighing it, the check in clerk made a point of asking if the pedals had been removed, the handlebars turned around (!) and the tyres deflated. Then we went back to the check in counter, she labelled it, I taped it up and took it back to the oversize luggage area and off it went. No drama, no additional charge. Points of possible interest: all of my flights have been to and from Heathrow terminal 4 and I'm a QANTAS Club member, which entitles me to an additional 10kg, which I've always exceeded when there's a bike involved. It's not just the bike is it? You take shoes, tools, then there is the actual weight of the box.......
Anyway, good luck and I hope you track down an XTC to bring back as a trophy.
Cheers,
pj