What it’s the best ddr4 ram and motherboard for under £250

Solution
These will work fine. You'll have plenty of RAM for any game. I like Crucial RAM as they manufacture it. Most other brands are rebranded memory.

PCPartPicker part list / Price breakdown by merchant

Motherboard: Gigabyte - GA-AB350M-Gaming 3 Micro ATX AM4 Motherboard (£70.98 @ Novatech)
Memory: Crucial - Ballistix Elite 16GB (2 x 8GB) DDR4-2666 Memory (£169.70 @ Amazon UK)
Total: £240.68
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2018-01-26 14:39 GMT+0000
These will work fine. You'll have plenty of RAM for any game. I like Crucial RAM as they manufacture it. Most other brands are rebranded memory.

PCPartPicker part list / Price breakdown by merchant

Motherboard: Gigabyte - GA-AB350M-Gaming 3 Micro ATX AM4 Motherboard (£70.98 @ Novatech)
Memory: Crucial - Ballistix Elite 16GB (2 x 8GB) DDR4-2666 Memory (£169.70 @ Amazon UK)
Total: £240.68
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2018-01-26 14:39 GMT+0000
 
Solution
Find your motherboard first.
It is your choice about size(ATX-M-ATX...) and features.
Then go to a ram vendor web site and access their ram selection app.
Enter your motherboard and you will get a list of supported ram kits.
ryzen is picky. You want a kit that is known to work.

And... do not plan on adding ram for an upgrade later. It is not 100% certain to work.
Buy all that you will ever need up front.
 
FYI. The Crucial Ballistix Elite is on the motherboards memory support list (MSL). Only the 4GB modules where tested for the MSL at 2666Mhz. Crucial's memory configurator says the 16GB kit is also compatible. I've never known it to be wrong. User reviews also show Crucial Ballistix Elite memory working at their true speed on other Ryzen boards even when not listed on the MSL.

Given all that. You should be safe with the 16GB kit working at the advertised 2666mhz when you enable XMP.