In theory, it is possible to buy a property to let without a letting agent; you are not legally obligated to have an agent. However, agents will deal with the tenancy agreement, the tenants’ references and any day-to-day administrative problems that might occur. Unless you are experienced, it is highly likely that you will benefit from the expertise of a letting agent.
If you do decide to take on a letting agent, it is almost certainly advisable to choose one who is a member of ARLA (the Association of Residential Letting Agents.)