Snowflake,
I would go for a Bergen, the new versions that have been used by British forces have the reinforced internal frame and also a reinforced padded belt that will carry most of the weight. Comfortable and allows you to load your gear in a methodical way. Also it has two side saddles that are part of the pack, great for storing must need items and if your using a handheld radio with headphone/mic piece will be able to store it in the top saddle pocket, you call on that if your also wearing a vest. Also the new Bergens like the new versions of U.S. Mil gear have the quick release straps, so unharnessing backpack from body to react to a threat is seconds.
I would look on U.K. Ebay, they have a few online sold by GI stores like Jim and the others have recommended on thread.
Back to work for me
Regards,