If demand for property is strong, the landlord may not be willing to negotiate. However, that's not to say you can't get a discount, especially if demand is weak (although that might suggest the property is overpriced to start with). I've twice asked for (and been given) a lower rent and once negotiated lower agent fees.
Your negotiating position will be stronger if the property is already vacant and you can move in quickly, or if the property is currently occupied and you want to move in fairly quickly after the current tenants are planning to vacate. Neither the landlord, nor the agent, make money on an empty property. Having examples of similar properties priced more more cheaply or even similarly may also you help negotiate the rate down.
If you look at sites such as Rightmove, you can see how long the property has been listed for and if the price has been reduced already.
Even without the above, sometimes a cheeky offer will be accepted if it isn't too far off the asking price. If you don't ask you won't get. However, unless the property is obviously overpriced or has some issue with it which would put off other prospective tenants, I wouldn't expect too much.
Agents are usually very reluctant to reduce their fees (they'd rather the landlord took a reduced rent!). Most times I've asked it's been refused (and I've walked away from properties as a result), with some agents incredulous that I would even ask, but as mentioned above, once I was successful.