I do not know about the UK, because it seems like no one in Europe can catch a damn break with pricing, but in the US, I have always waited until the newer model is released and then watch for deals at B&H Photo or Adorama, sometimes Best Buy. If you can wait, and if this is actually an option for you, it does two things -
1) You get to see how much a new iMac is going to cost and whether the update is worth it
and
2) If it is not worth it or the cost is too high, you can get a 2017 for a significant discount.
Again, this is predicated on a retail chain in the UK similar to B&H discounting older stock to get it out the door. My intuition tells me that is not generally the case and that I am very spoiled in that regard. If not, then I would get a 2017 iMac if you really need it now. Otherwise, I would wait for the 2019...
I see you have an Apple Refurbished Store in the UK, but that iMacs are out. You might want to check into the cost once the Store is replenished. I have had great luck with refurbs and Apple Care is an easier cost to justify when you buy a refurbished unit.