I've had two GTT's (because my baby is measuring big).
The first time (at 28+4), I had to see the midwife in between the two hours because I hadn't seen my midwife in the past 7 days. I didn't have a scan.
The second time (at 36+1), I didn't see a midwife or have a scan (but I had a scan the day before which was why they sent me for the second GTT).
I was told I couldn't walk around because it activates your metabolism and can affect the results, but I was allowed to drink water.
Just make sure you book it for the morning! My first was at 9am, my second was at 2pm and it killed me not eating on the second one (especially as the receptionist staff were sat eating their lunch when I got there!)