dude...
"Commons side effects of using amitriptyline are dry mouth, loss of appetite, drowsiness, muscle stiffness, nausea, constipation, nervousness, dizziness, blurred vision and insomnia." these are the more common side effects...
"Some rare side effects include tinnitus, hypotension, mania, psychosis, anticholinergic effects, heart block, arrhythmias, lip and mouth ulcers, extrapyramidal symptoms, depression, and hepatic toxicity." those are the rarer side effects for that drug he's prescribed you...
Dude I'd almost certaintly say that he's prescribing you the wrong meds (FOR YOU). That drug may work for some people, however, apparently your body doesn't like it... at all.
If you are really having migraines (I've had 'em so bad I cannot even see; my vision is like "snow" on a dead tv-channel), you could try figuring out what they are from- probably stress.
But I mean, if this doctor is too goddamn stupid to see that you're experiencing extremely uncomfortable side effects, he ought to know better than to "increase your dosage". Is health-care public in England? If so, I'd probably at least consider trying to find another doctor, visiting a "walk-in" clinic, or even going to urgent care (they'll provide you with a solid solution to your problem, though you'll have to probably wait 5 or 6 hours to be seen by anyone).