Note: I do not own Traders and Gunboats, hence cannot speak from a rules perspective, only comment on the tech/physics involved.
The drone does penetrate the hull before detonating, hence its spike like configuration.
Which means the laser effect is pointless. If you're already inside the hull and setting off a nuke....would a laser head really make things much worse?
I'm not convinced the result would automatically be kablooey, especially on a big ship. The internal bulkheads would be closed in a combat environment and the major internal bracings may well be made of the same stuff (e.g. bonded superdense) as the primary armour belt.
The other problem I have is that a 20 dTon small craft ramming a big warship's armour is going to end up pancaked.
If you set off the nuke before impact with a BPL, no problem.
If you set of a contact nuke at impact, also no problem.
If you want to "drive deep into its hull", forget it.
This isn't a bunker-buster punching through rock; the armour is made of the same material as the drone and is very, very thick. I don't doubt that it can be penetrated with enough force - steel bullets can penetrate thin steel sheet, after all - but I'm really not convinced it'll happen in a way that's going to leave any device in the guts of the drone as temperamental as a nuke capable of functioning.
Personally I'd just stick with using the rules for the humungous bomb-pumped laser.