Unless you live in a Conservation Area or a National Park, you would be very very unlikely to require Planning Consent. This is because you have certain 'permitted development rights', which means you can make certain alterations without needing consent. If you live in the above listed designations, your permitted development are generally curtailed.
To be absolutely sure, trying calling the customer centre of your local planning authority and ask the question - they MAY give you informal advice for free, though many now have 'formalised' even this service and charge a nominal fee for simple advice.
What you will need is either:
Building Control approval or
a contractor to fit the window who is part of the 'registered window installer' system
This is because it is imperative that you do not have fitted a window that would degrade the overall thermal performance of your property - part of Government desire to save energy. All reputable door and window companies will be part of the scheme - most general builders will not (as it's too specialised an area for them to gain the certification).
Planning Consent and Building Control Regs are two separate things.