Panel doors at B&Q are too big for my door frames?

I want to replace the old battered doors in my house. I have just measured the current doors and they measure up as 1960 mm x 750 mm. The smallest door at B&Q measures up at 1981 mm x 762 mm - on the description it says that there is a 4mm trim allowance on 2 long edges, 6mm bottom edge and none on the top edge. My door frame would need 21 mm taken off of the height of the door and 12 mm off of the width, so going on these trim allowances, the B&Q doors will still be too big. Does anybody know where I can buy cheap 4/6 panel doors for my door frames - am I right in assuming that when you measure up for a new door, it is the size of the current door that you measure up and not the width and height of the door frame? Any advice appreciated. Thank you.