The planning restrictions are such that you can have a dish up to the size of 90cm without gaining planning consent, providing it doesn't protrude higher than the highest part of the roof (excluding the chimney), a 45cm dish can be mounted above the highest part of the roof, but must not be higher than the highest part of the chimney.
A 90cm dish can be mounted on the chimney, but only provided that it is not higher than the highest point of the roof.
You can have bigger dishes than these, but they require planning consent, if you are in a consevation area, or live in a listed building, then separate consents are required.
Also look here.
http://www.odpm.gov.uk/stellent/grou...an_606331.hcsp