Ben – I rode from Exmoor to the Peak District on an S-Works Roubaix… I replaced the seat post with an alloy one and stuck a Topeak seatpost mounted rack on there. It’s something like this, but they do a broad range that really need to be looked at in detail depending on your bike. They do a three part pannier (top and two sides) bag that was fine for ligthweight touring. That combined with one of their top tube mounted Tri bags (it’s only small but surprisingly useful) did the job, but a bar bag would help too.
I still use the rack / panniers for my commute as I like the way I can leave the bike in the warehouse at work and just unclip a QR clamp and take the whole lot of my gear with me.