Nothing to do with Brexit, don't even bring that into it.
I can sell the 4690K for £120 right now and get new i7 4790K for another £230.
To do the same swapping a 4 core 8 thread on AMD side taking your prices in to account would cost me £250 for the CPU, £120 for the Board and £80 for the DDR4 RAM, £200 more and I would only get about £90 back for my Z97 and DDR3.
So just to swap to an AMD 4 core + SMT would cost me £100 more than going the Intel rout.
Besides all of that I would not reach into my pocket for £200 just to get SMT.
This is the problem AMD would have with pricing like that they ain't going to win any last platform Intel users over, few Sandy Bridge and Fewer Ivy Bridge, their main custom would come from existing Piledriver and Bulldozer owners, of which there are not many.
For Zen to be a success it needs to win Intel users over, with prices that are token cheaper than Intel they can't achieving that.
Edit: add £75
to replace this going AM4
Its not relevant what you paid for a CPU years ago when the pound was $1.5ish - its $1.25ish now,so a direct replacement will cost more.
The Core i5 4670K and Core i5 7600K is a $242 SKU,so expecting AMD to pay for currency fluctuations is hilarious.
AMD is only going to care about what Intel has out now,and as the Phenom II indicated they didn't massively undercut Intel and AMD still sold enough of them since the Core2 quads were not really significantly better.
Look at Piledriver - AMD pricing was not massively lower. 6C/6T FX6300 against a 2C/4T Core i3,and the 8C/8T FX8300 series against the 4C and 4C/8T Intel Core i5 and Core i7 CPUs.
When AMD had the Athlon XP and Athlon 64,pricing was not massively lower for many SKUs when compared to Intel.
AMD has far less of an image problem with CPUs than with graphics cards,and the only reason they have had to price them cheaply is since they lacked performance.
If they have enough performance they will price them higher otherwise it defeats the point of them making Ryzen.
Intel is selling plenty of Core i5 7600K and Core i7 7700K CPUs at the current RRPs worldwide just like the previous Core i5 6600K and Core i7 6700K.
8C/8T Ryzen at £300 to £350 will be 8C/8T against 4C/8T for the same price and the reviews will show that.
In the end,if AMD offers BW-E level IPC,can can get Turbo upto 4GHZ for £240 to £250,it will still be £100 cheaper than a Core i7 7700K unfortunately for you.
I would be quite happy to replace my Xeon E3 1230 V2 for that price,on a new platform and without needing to splurge on a new cooler.
If they are going to price it a bit cheaper I would be happy,but people need to be realistic - remember what happened with the RX480 for example??
AMD has started to realise,Intel and Nvidia have stood firm on pricing and its helped them out- in the end people who don't want an AMD CPU won't buy one even if Ryzen was half the price and double the performance. People still bought P4 CPUs even when AMD was just better in most metrics.
Edit!!
Plus in the end if you are already on an older platform its just cheaper to upgrade your CPU anyway. AMD and Intel cannot compete with secondhand CPUs,but the people who are more likely to buy secondhand are not really relevant to Intel and AMD TBH.
I would probably source a Core i7 4790K from a secondhand parts retailer.