Hello "anonymous", I have read this exchange with interest ...
- If I have read it correctly, you may not have "broken the law" at all; you have simply added a directional antenna to an existing piece of "legal" hardware without modification?
- if so you may be "legal" as you are still "transmitting" the same amount of power; that is your "box" has no knowledge of its antenna so carries on as if you still had the original whip (?) antenna attached.
What your new antenna does for you is to focusing most of that transmited power onto the tower and decreases the aperature of your receiver's "visibility" to a circular locality closer to the tower, thereby increasing its sensitivity.
What matters is your Effective Radiated Power - look it up:
As the OFcom spec does not give ERP (or gain over EIRP) only a power out of the transmit section of the "mobile" (maximum permissable is 2W for a Class 1 device and 250mW for a class 2 device) it becomes difficult to decide if changing the antenna will constitute "breaking the law". It will all depend on the efficencies in the system and the class of the system you are altering the antenna on; for example if you are using a class 2 device (probable) and you use an antenna with 10dB gain over isotropic (ie something like a 4 or 6 element "yagi") then you are probably still "within the law" as you are now only radiating the same power as a class 1 device. Interestingly, you will have increased the range of both the transmission distance and reception distance of your class 2 device by a factor of about three times, which, from your description of what you have done, sounds about right?