When I have to sign such forms for my patients, once I have identified what kind of form it is the patient is told what the charge is and normally told to return in a day or two NOT 6 WEEKS!
Any form which only requires 15 mins or so of my time should not take this long.
At the very least this GP should feel a little embarrassed by the whole situation.
As mentioned above, inform the practice manager and GP of the situation and that it is imperitive the form is completed ASAP.
Find out when the GP finishes on your chosen day.
Forward them of your visit.
Go a bit early and collect in person.
Stay calm and play the damsel in distress card.
The sympathy card works better than the threatening one.
If the GP is still not sympathetic they obviously are lacking in any compassion and wouldn't respond to threatening behavior any faster.
Hope it all goes well.