Thanks, this is the type of thing I have been looking at, but I have been told not to go for the higher power motor, so to keep legal.
In case you don't know the rules to be legal, here's my guidance:
1. The motor must be rated at 250w. That's not how much power it makes. It's basically a formality of marking or listing for compliance. 250w motors can make anything up to about 3kw. That means that not all 250w motors are equal.
2. The power must stop when you stop pedalling. You can have a throttle, but it too must stop giving power when you stop pedalling.
3. The power must stop whenever your speed is above 15.5 mph.
4. There needs to be certain information on a label on the bike where it can be seen. There are some options, but basically, that's manufacturer (of the motor if a conversion), max speed (15.5 mph) and power (250w).
I don't believe the police care too much about whether throttles are independent or not. I've even seen them using an independent throttle to test the speed, and when the power cut at 15.5 mph, they said it was OK, though, as above, the bike is non-compliant.
There's no practical way to test power, neither is there a legal maximum, so you can push as much power as you want through your "250w" motor as long as it doesn't burn. It must be marked 250w.