If I were you, Ghost, I would think about typing out a letter of your own to the consultant. In it I would put all the things you mention that the GP hasn't mentioned (vit D, calcium, growth.)
I would document all the symptoms, how long they've been going on for, and what's been done (and hasn't worked).
I'd lay it all out in a really easy to read way, so that when you go into to see the consultant, you can give it to them to read. That way, they will be getting a fuller picture. I would include the family history of coeliac and your ulcerative colitis too.
I think that the consultant will suggest testing for coeliac antibodies (among other things probably). But just in case they don't, I would be prepared to ask for that. In support, I would also print out these NICE guidelines about testing for coeliac disease and highlight all the relevant points (the list of symptoms matches your DC's symptoms on several points).
If the 'not anaemic' thing is mentioned, I would point out (politely) that the range of symptoms that coeliacs have pre-diagnosis is very wide, and each individual has a different presentation (and some have no external symptoms whatsoever).
I would also read up on what Coeliac UK say about diagnosis so that you're prepared.
I can't imagine that the consultant would refuse the test - there may be other things they also want to test for, which is sensible. I would also make sure that it's clear to you when the test will be done, and when/how you will get the results.
For a young child, blood tests are usually done at a hospital (rather than at the GP) by someone trained in taking bloods form children.
Good luck and let us know what happens.