We all want wider rims, for stability, for the look, for our cravings...nevertheless, there are some legal rules to follow:

1. the tyre tread must be within the mudguard or guard extension. The side wall of the tyre can protrude outside the guard but not the tyre tread.

2. you cannot increase the tracking width anymore than 25mm without obtaining a low volume certificate. 
   

You can install fender flares on your car to extend your guard/wheel arch so wider rims can be accommodated, or you can roll your guards to make the wheels fit. In either case, you will need to seek professional assistance.