Since you are in the UK, I would suggest checking out the various European manufacturers and UK based retailers like Cotswold and visiting websites like this one-
Outdoors Magic

For a list of manufacturers-
Exp Gear

Also, with the dollar pretty worthless at the moment, packs from manufacturers like Granite Gear or Gregory would be worth a look.

You can get a really nice pack for less than $400. Personally, I wouldn't want to spend more than about $250 if I was buying a new one.

Don't forget about eBay. Plenty of good gear there if you know what you are looking at, but lots of cheap, poorly made gear too, so be sure to do your homework before bidding on anything.

One thing to keep in mind-this is a lightweight backpacking site; some of the suggestions you may read may be recommendations for ultralight packs. BUT, as you probably already know, traveling with a pack is not the same as backpacking or hiking.

I would be very wary of traveling with a UL pack because I doubt most of them could take the abuse a pack gets while traveling. Get something sturdy, even if it means a bit of "extra" weight. It will be well worth it when traveling and you find yourself tossing it into the back of a truck or car, loading it into a bus or watching it disappear down an airline luggage conveyor belt. Anyone who thinks you can travel with one of these one (or maybe two) pound wonders, probably hasn't done it.

If you haven't had the experience, stand at a window sometime and watch luggage get loaded into the cargo hold on a jet-not a pretty sight.