If the only reason for a couple not being free to marry/enter a civil partnership is that one of them is awaiting a divorce/dissolution of a civil partnership, entry clearance should not be refused for this reason alone (though ECOs would normally expect to see some evidence that divorce/dissolution proceedings are well under way)
So I guess again, if you have a strong case & belief that your annulment will be granted, one can try as it says you can try as long as you provide evidence that annulment proceeding has started. Also if one has a spare 500 quid hehehe