If you're trying to make a margin on growth vs. cost of borrowing then maybe you should ask your broker about options/shorting. If not, then what you're considering doing is a really bad idea.
There're some brokers who offer credit facilities but it's only provided to wealthy investors and you have to have some assets (like property) to guarantee your ability to cover losses.
If you a small investor, like most of us, than you could try financial spread betting and CFDs as you trade on margin and thus effectively borrow money from the broker.