Please let us know what response you get from any messages to PSWP or Yose, otherwise all you can do is search through Amazon and Aliexpress for a 250w rear 27.5" freewheel motor kit. If you find one that looks OK, show it so that we can pass comments. One other point is that some of the cheap kits have really bad controllers that have a delay before the power kicks in and they run on for a bit after you stop pedalling. They're better than nothing, but I think it's better to pay more for a better control system.
Another possibility is to build your own wheel, either yourself, a friend or a local bike repairer. You can buy the motor for £70, the controller kit for £50, the battery for £150, the is spokes for £10 and the rim for about £15, which is about £300 or 350 Euros. A repairer would charge about 50 Euros to build the wheel, though most of them aren't setup to deal with motors. You can learn to build a wheel yourself. All you need is a spoke spanner that costs about 10 Euros, a brain that can figure things out and some good instructions, which you can find in Youtube and here:
A tutorial on how to choose parts for, build and true a spoked bicycle wheel
www.sheldonbrown.com
You use an on-line spoke length calculator to get the length of the spokes. I recommend 14g spokes in a 2-cross pattern.