Generally spoken, for such a scenario (high bombers over the base at at takeoff) the 109K is the best choice, as it offers the best climb performance (having 10 instead of 5 minutes of WEP is a significant advantage over the Spit 14)
But as Karnak pointed out, Mossies at 12k are pretty rare. I would call that a suicidal altitude for a bomber with no defensive armament whatsoever. There's actually a good chance someone already at altitude will kill the Mossie before you catch up.
However, if the Mossie flies at or near optimum altitude (~27k), you can forget it. Let me give you an example:
A Mossie carrying the 4k cookie bomb, but no external stores will fly at
-15K 377 mph
-20k 372 mph
-25k 385 mph
-30k 380 mph
with their optimum alt being 27k
Assuming an altitude of 25k: The 109K-4 takes 9 minutes to reach that altitude, during which the Mossie will have moved ~58 miles, thats over 2 sectors while the 109K has covered only ~22 miles over ground during the climb. If you took off while the Mossie was directly overhead, the bomber is now ~35 miles, about 1 1/2 sectors, ahead of you. Not taking into account the time the 109 needs to get up to speed, it would now take (theoretically) ~40 minutes(!) to catch up, during which the Mossie would travel another 10 (!) sectors
This shows that in case of a Mossie flying at or near optimum altitude, you either need to take off way ahead (giving the trend of just bombing a town center, this may be diffcult to impossible), or you need already to be airborne at alt and basically stumble over that mossie.