Last year got BT Fibre for my gf's house, got an install date but then it was put back 6 weeks, turned out to be the cab full, the order never failed.
I can't say for other ISP's but if we (Sky) try to place an order on a full cabinet it fails immediately. The DSL checker will say 'wait list' so I'd assume some ISP's may allow you to wait, but not all.