Bridegstone S01 UK shortage

I am looking for 2 Bridgestone S01 255/45 ZR17 tyres for my 38. I am in the UK and I cant get them from anywhere (actually most quote the Y rating and not Z which presumably would be ok). Most fitters say they are on back order with Bridgestone with no expected date.
These are for the rear and I have S01's on the front as well but these are fine. Question is, I dont really want to mix tyres but seem to have no choice as it would be very expensive to change all 4. Can I mix a new set of rears with the existing fronts and if so does anyone have any recommendations.