Starlight well, firstly this is your body and your baby and so you must do what is right for you and you may not agree with my views and opinions, which is fine, but I did shedloads of research on GTT, GD and BMI etc so hear are my views:
Firstly, growth scans are notoriously inaccurate and apart from telling you whether baby is head down or not, serve little purpose apart from scaring pg woman half to death. They are generally used as a scare tactic to persuade 'difficult' women to go into the CLU (oh, your baby is likely to split you in two/get stuck as it's soooooo huge). Woman has pregnancy hormones and so gets scared. They told me DD would be over 10lbs (she was 7lbs 15oz and DS would be at least 10, probably more, he was 9lbs 3oz after a section so birth weight inflated by drug fluids).
there is good research that says late scans do little to improve outcomes and can contribute to medical intervention.
With a high BMI and mine is much higher than yours - you are only marginally over their cut off BMI - it is even harder to get a good estimate of weight from a scan. i believe no one grows babies they cannot birth due to size alone.
Secondly (my views are a bit off the wall) I do not believe that GD really exists. i have done plenty of research (google Michel Odent) and there is evidence to support the chicken and egg theory. If baby will naturally be big, needs more fuel to grow and sugar spills out, whereas most HCPs believe GD spills sugar and makes baby grow bigger than it should.
The theory by Michel Odent if that excess sugar is a natural by-product of pregnancy and at your stage, probably does not indicate GD. The tolerance limits for GD vary worldwide, so you would be considered diabetic in one country and not in another.
True diabetics in pg tend to be near the edge of diabetes pre-pregnancy and getting pg tips them over. These women show ketones early in pregnancy and spill sugar by around 15 weeks - far earlier than GD tests.
This is YOUR choice and YOUR baby/body. If you want to have the GTT then do but be aware if you show raised sugar levels they will not let you into the MLU (or say they will and find a reason to t/f during labour). If this were me, i would refuse the scan and GTT and if they pressed me say thanks for your advice, DH and I will consider it on a weekly basis. If you start showing urine in your sugar, then I would have the GTT.
If they start really hassling you, smile and politely use broken record. Thank you but we are aware of the risks and they are acceptable ones for us. Providing no sugar is detected in urine, we do not wish to have scan/tests.
If you do not have a GTT they cannot say for certain that you have GD.
Plenty of women who were told they had GD have birthed at home, so the MLU should be fine (have you considered a HB btw, it is your right - no one can force you into MLU or hospital) as medical care is nearby. You could always switch to a diabetic diet anyway - it can prevent pre eclampsia apparently (oats, no refined sugars, low GI etc)
If they continue to hassle state you consider it harassment.
These are my views and some will disagree but you need no further tests unless you want them.
HTH.