A 90 L bag is huge and far bigger than what you likely need. I carried a 60L pack in NZ and it was fine for extended camping. Osprey and TNF are American brands. Nothing wrong with them, but I would look for something from the UK or the EU, like Deuter, Bergen or Jack Wolfskin for value for money. Have you looked at Cotswold's website? They have a lot of gear, including all of these brands. The big thing with a pack is fit. Not all packs fit the same regardless of size. I had a huge Arcteryx pack that never fit right, so got rid of it after one trip. An ill fitting pack will be miserable regardless of what you pay for it, just like boots or shoes.

Your tent can be tied to the outside of your pack. As far as clothes go, 3 sets could mean anything, so not sure what you are taking. Take a couple of sets of base layers and one set of outerwear and insulation. Go through your clothes and gear list first, then decide what pack to get to hold it all.

If you can, follow Bill's advice and go to a well stocked store with your stuff and see what fits. A really big pack is far too tempting to fill up with stuff you don't need, unless you are winter camping (as in snow on the ground winter) and the space is for lightweight insulated clothes like a big down parka (like in my photo).