Confidence: It might be this.
Notes: This could, I think, only be this species or a male of Bombus lapidarius, the Large Red-tailed Bumblebee. However, I think it would be biologically impossible to have a male of B.lapidarius in this country in early June. I suspect the reason that other people have been wary of jumping in is that there is usually yellow visible at the anterior end of the abdomen (gaster/metasoma) in B.pratorum. Also, we can't see the head. I must say that, taking scale from the Thrift flowerhead, it is a B.pratorum of generous proportions. I'd advise against recording or publishing this.