Welcome to the board. Your story is all to famiiar to many of us.
If I remember correctly you can help with gluten-free foods in the UK if you have an 'official' diagnosis. I don't know how it works but if that is vital to you then do stay on gluten until all your testing is complete
If you can ask to be put on a cancelation list for the biopsy. Then if someone else cancels their appointment at the last minute the doctor can slide you in. You could also try asking the doctor if a return to negative on the blood work (if that is positive) along with resolution of symptoms on the diet would be enough to diagnose. Since they won't biopsy if the blood work is negative and the blood work is already drawn you could then start the diet right away.
There can be false negatives with both blood and biopsy so you do need to give the diet a strict shot after testing is done no matter what the results.