A handful of our garden bumblebees, including the white-tailed and buff-tailed, are able to forage for pollen and nectar through the winter months. This is in part due to our increasingly mild winters and also the wider variety of winter flowering plants we grow in our gardens. In terms of phenology (the timing of natural events) the appearance of the first bumblebee queens has been getting earlier but this would still be well into March. Your sighting is probably a worker from a nest that is 'ticking over' rather than a queen emerging from hibernation.
I think the Bumble Bee was a buff tail but I didn't get a photo. It's interesting that Bumble Bee's can forage in the winter ( like Honey Bees?); I had not realised that the workers can over winter. However the Bee I saw was very large the size I associate with Queens.