The “don’t mix kibble and raw” is a myth. Racing kennels, at least here in the US, feed a mix of raw meat, cooked veg and/or grain, and kibble. Sounds like the UK kennels do as well. I have fed mostly raw with 7 greyhounds over 17+ years, sometimes mixing raw and kibble, with no problems, even with my IBS dog who could not eat plain kibble at all. If your dog likes his add ins, go right ahead