The GFR issue means your kidneys aren't filtering properly and only working at 55% (currently). This is of concern, and I believe NICE recommends not to prescribe Metformin when there are known kidney problems as it can make matters worse.
Have you tried to control your glucose levels by diet? If you give this a go you may find you can manage with a very low dose of Gliclazide. Yes, you do have to self test before driving, and every 2 hours during a long drive, but if you want to take this disease by the scruff of its neck and control it by diet, you need to self test anyway.
When I say by diet (with the aid of some Gliclazide) I mean really cutting down on carbs