Hi,
If I could add my input perhaps.
Andrew is correct about the speedo readings, but I think he may have been using 215 45 tyres not 225 45 tyres and so that affects the width a fair bit.
The speedo difference between your true speed and the Speedo reading will be very close. currently with OEM tyres on your speedo will read something like 53 MPH at a genuine 50 MPH. With tyres of the diameter you are considering you will be doing 53 MPH rather than 50, so your speedo will be more accurate.
BUT, the tyre width of 225 WILL give you problems, it will rub on the wheel arch and/or inner wheel tub (depending on the ET of your wheels)
I managed to squeeze 215s on my A2 and they did JUST rub occasionally and I had to grind away some of the inner arch plastic to get more clearance and they also rubbed slightly on the inner wheel tub, so 225 is not going to work unfortunately.
Steve B