Welcome to the forum. Re deposit, that's likely to be because they're bullsh!tting on the expected delivery date. Search for marmite's post on his expected M Sport delivery date. On iPhone so can't search at the moment.
Re discounts, I was told no way by one dealer but ended up with quite a good discount from another so shop around and make sure they know you're ready to order and pay deposit there and then. This might sound obvious but you're much more likely to get a discount after spec'ing a car with them and then just before signing slipping in an another option that you were always going to include anyway.
Might be worth posting your spec here too in case you missed something
Good luck