If you want all the satellite and all the transponders it's impossible!
A lot of the beams do not focus on the UK and some are just not receivable here (most you probably wouldn't want).
If you want the main positions that most UK users receive then 1.2 meter would be a good start. Remember you will need planning permission and have to consider the motor a DiSEqC one would normally not be powerful enough to drive a dish that size.
The position of the dish will also have to be considered. The wind load on a dish that big will restrict where you can put it.
Unless you want a specific channel satellite position.