There's a difference between Outline Planning and Detailed Planning. One accepts the principle that a development is allowed to occur, the other shows the exact elevations proposed and the use of materials for walls and roof. Once a structure has been in place for >4 years without the necessary planning, a Local Authority cannot force its subsequent removal.
You should be guided by your solicitor, but he/she may suggest applying for a Certificate of Lawful Development. This is often used in these cases to 'regularise' the planning situation. I don't think that you can get indemnity insurance to cover this.
The Redrow situation is almost certainly caused by a restrictive covenant placed by them on a houseowner developing their house for a period of time (often 5 years) after purchase. This is quite common on new estate developments and it is done to ensure the developer has control over what happens whilst they finish and sell the rest of the dwellings on the estate. They don't necessarily want homeowners doing extensive building works. Usually it requires permission through submission of plans and paying a smallish fee - perhaps �100 - to Redrow. You will almost certainly have to do this retrospectively because the buyers solicitor will expect it. But your solicitor will advise.