I moved on from the Blackburn racks, as the platform is a bit small, and kinda oddly placed (for me).
I use the Racktime Topit racks on most of my bikes now. They’ll take small panniers, but I only ride for the day, so everything I need goes in a drybag on the top.
Or – in a drybag, in a Wald 137 basket (don’t knock the idea of a basket until you’ve tried it 😉 ).
I’ve got a Swift Ozette rando bag, but it’s not a great fit with those racks – sure it works, but only ‘ish, I can’t get it to feel especially secure – it does fit almost magically well on a Nitto M18 tho’ (as does the Wald basket). Other rando bags might work better / differently I guess, but everything I looked at ‘needed’ that tombstone on the back of the rack to lock down onto securely.
That said, a Porteur bag might work ‘better’ than a traditional rando bag?
UK supply might be a challenge, but Pass & Stow racks came up a lot in my research. There seem to be more options now than when I was looking, and they’ve been covered above – Specialized Pizza Rack (supply bumped on to Feb 2017 I think), plus the Surly and Pelago porteur racks.