You don't need an EN15194 cert for a bike to be legal in UK. All you need are:
- Motor RATED at 250w or less
- Assistance cut above 15.5 mph
- Assistance stops when you stop pedalling
- A label that shows key info.
- The bike can be pedalled without power
Obviously all the regulations that apply to non-assisted bikes also have to be considered. Technically, the max voltage allowed is a 48v battery, but that's not tied down tightly enough to argue either way whether you can go higher.
I only looked at one court case, where the legality of the bike was in dispute. The Judge simply went through the above 5 points. In that case, it failed on the last point.