Which rating?
Dear OP, I can offer some advice on the IMC/IR(R) rating as had it added to my licence fairly recently. The advice is: expect delays. At least a week on top of the usual turnaround times will be required, as all the documentation now needs to go through the Department for Transport, due to some legislation issues which manifested themselves post 8 April 2014. That won't apply to any rating other than IMC/IR(R).
Apart from that they seemed to be sticking to their 10-day turnaround time, in spite of a wave of EASA conversion applications (i.e. I had my licence back after 17 or 18 working days, don't remember exactly).
Hope that helps,
/h88