A very quick measurement on a 1/2mil chart, of Gatwick, Stansted, Luton, Birmingham & Bournemouth zones show them to be based on a 30mm rad. circle, which is 8 Nm in real life, centred on the airfield.
Two lines are then drawn, parallel to the main runway and these lines create the edges of the zone. Each zone is a slightly different width, but approximately 9-10 Nm seems to be the norm.
The final shape appears to be negotiable, Gatwicks northern edge is cut away on a east-west axis, convieniently following the railway line whereas Luton's northwest corner has also been chopped away. Calais has a marked assymetric zone