Sure, backpacks are great, but have you ever tried riding a bike with a bag on your back for hours? It’s not all that much fun and can be a one-way ticket to back pain if you try and carry too much. That’s why frame bags are pretty much essential for any bikepacking trip. You have to pedal the bike, so why not let the bike do the carrying too?
Bags come in a host of different varieties, from those that fit into the main triangle of the frame to others that hang from the bars, are mounted on the seatpost or even small ones that sit on the top tube. Before you buy any, though, make sure that your frame/finishing kit (that’s bars, stem, seatpost, etc) are rated to carry extra weight. The last thing you want to do is damage the bike by hanging too much weight from it. This likely won’t be an issue with a steel bike, but if you’re using a carbon one then definitely check.
The other thing you’ll want to do is make sure that those bags are waterproof, especially if you’re bikepacking in the UK. Even with mudguards, riding in the rain can thoroughly soak everything through and you really don’t want to get where your heading and find that everything you’ve brought is sodden.
Both Pro and Madison offer a range of bags for your bike/frame, and remember that panniers can be just as helpful (and even more so if you’re taking a lot of stuff as they generally have a larger capacity than frame bags). Make the bike do the work, not your back.