Well, you've certainly given us the background - might be nice to fill in your profile a bit too.
Having said that, what you're looking for is a *large* pack to carry a heavy load - there are probably
too many options in that category to name, and as this is a lighteweight forum, many of us simply don't do huge backpacks with 20 kilo loads in them. True, I do in winter and I have a large MEC brio bag that's
tough as nails for that - however you likely won't have that option available for purchase in the UK.

My advice is the same as to every beginner pack shopping - put what you're taking in a bag or box - go to some reputable outdoor stores and try the load on in the pack - buy what is quality and comfortable to you - those last three words being a lot more important than anyone's opinion you find on the internet.