I note you already have cotoneaster horizontalis on your list; I can only add that, this spring, mine was absolutely covered with tree bumblebees, to the exclusion of other varieties. I wondered if this relative newcomer was excluding other bees from visiting this plant?
Hi, having trawled through my records for the past 3 years, here are some flowers B. hypnorum has visited that are not yet on your list;
Clematis alpine (blue and purple), rosemary, crab apple, dog rose, hebe, Cotinus coggygri, Salvia purpurea, Astrantia, Teucreum (Wall germander), pyracantha, rosebay willow herb.
This year I began to keep records about the types of flowers bees are visiting so I am pleased to have the opportunity to use some of this information already!
So some additions to the list
Bramble - Rubus sp
Figwort - Scrophularia nodosa
Herb Bennet - Geum urbanum
Lousewort - Pedicularis sylatica
Oil Seed Rape