As all PP posters have said, that ou can either sell without PP at a lower value, or get some form of planning and sell for a higher value.
First of all speak to a land surveyor, they will have an idea of the cost of land, potential etc, and give you the best ideas for the plot. If you get the plot valued now without PP you will know where you stand in terms of value.
If you want to proceed with planning, get in touch with a Architect. If you think it will be complex, challenging seek advice from a planning consultant. Most developers use planning consultants, to find poop holes and fight their corner. They will act as your agent, or to reduce costs find an Architect who has a good relationship with the Local Authority, and has a history of obtaining planning approval for similar sites.
You could either apply for full planning permission or outline planning permission. IMO I would only apply for outline, with all matters reserved except siting, access. This gives you an approval, and increases the value of the land. However the design, style, scale of the properties is then decided by the Purchaser and approved through a discharge of reserved matters application. This is less “work” for you, and doesn’t restrict the site which is key to a developer. (They can use their style of house types etc)
Bare in mind you may need other consultants for the pre-application stage, but these should be arranged by your Architect who would give you an idea of costs and obtain quotations.
These could include; tooographic survey, ecology survey, highways consultant.
Outline permission is valid for 3 years from the date of approval.
The only thing I would say is that if you apply for planning permission and the application is refused, this may decrease the value and desire to the developer. Unless it is refuse on grounds they think could be overturned.
The other option would be to have a pre-application meeting with the planning department. You may have to pay for this service. But the information you show / discuss is not made public. This way you could gauge the Councils reaction to a development on the land, and potential be aware of any hurdles you could face.