SU85SW represents the south west quarter of the 10km grid square SU85. You need to identify the coordinates of this quadrant's top left and bottom right corners. You could then put these coords into a converter such as this: https://www.ordnancesurvey.co.uk/gps/transformation/
This webpage provides useful information on how to understand a grid reference: https://magic.defra.gov.uk/Help_Working.htm
In your example, the bottom left of SU85 is at 480000,150000 (The SU defines the 100km square at 400000,100000, and the 85 indicates the 10km square within this.) The top left of its south west quadrant is 5km north of this, and the bottom right of the quadrant is 5km east. This gives:
Top left: 480000,155000
Bottom right: 485000,150000
You can then plug these numbers into the OS transformation tool (use height = 0), to get:
Top left: N51° 17' 18.69", W000° 51' 14.98"
Bottom right: N51° 14' 34.27", W000° 47' 1.19"
EA Data Sharing & Access Team