The 'Going for Bust' charity shop has a single door, but the building carries both "Institute Road" and "The Square" road signs. It's postal address is 11, The Square, however. I'll also be accepting "RNLI", since although there's a single street sign on the RNLI shop ("The Square"), it does have a single door - Going for Bust, as someone has pointed out, actually has a double door. The clue should have referred to "a single doorway ..!

Going for Bust is a Breast Cancer Charity, dedicated to raising funds through its Charity Shops, for local Hospitals and Support groups, in the fight to combat Breast Cancer in the East Dorset and West Hampshire areas. Believed to be the region's first and only Breast Cancer Charity Shop. The charity was set up by local people, who have been directly affected by Breast Cancer, who felt that Charity Shops offered the best opportunity to provide both sustained and substantial support for people who live in the area as there was a clear shortfall from funds supplied by the NHS.