Paddock next to my house mainly ungrazed grass with some wild flowers and several flowering thistles
No interactions present.
The underside of the abdomen is covered in pollen, giving it the "fluffy" appearance and the distinctive way of foraging. There are pollen-collecting hairs (scopa) on the underside of the abdomen in these Megachile (leaf-cutter bees), and also in the Osmia bees so you weren't far off with your ID :)
Lat/Lng: 52.2461, -4.1773
OS grid ref: SN514631
Edge of village approimately 2.5 miles from the West Wales coast.