In theory this sounds like a good idea but in practice it will depend on the seller.
If the seller is in an upward chain (they need the money for their next property) then a cash buyer, offering the same price, will be more appealing than a buyer who has to sell their property as there is less chance of the chain falling through and them losing the purchase of their next property. However if they are in a chain then they may not be willing to take a lower offer due to them requiring the cash for their next purchase.
If the seller is not in a chain then they might hold out for a higher offer, whether that be a cash or mortgage buyer.
If the seller requires the cash instantly to pay off encroaching debt, for example, they they may be willing to accept a lower offer in order to quickly secure the funds.
So, to summarise, you may only get a better price paying by cash if the seller needs the money quickly or is willing to take a lower price for less chance of the chain falling through.