I have got 241 ums marks at AS. To get a B in A-level, does it mean i need 179 marks i.e. a high D?
Yep, since the minimum UMS mark needed for a B at full A-level is 420/600, so if you already have 241 from AS, you only have to get 179 at A2.
for AS (300 Max)... its 240/300
for A2 (600 Max)... its 480/600