but under uk law i am allowed 250 Watts continuous
As an aside... what particular UK Law are we talking about here? Is this something akin to the Disabled Mobility Scooter kind of installation where politicians have decided you can't give little old ladies trundling along the pavement a turbo boost for fear of over-stressing their heart pacemakers?

250W continuous is kinda ambiguous... define 'continuous'... kinda says nothing about 'peak' or how long that 'peak' could last for... you could design something that was 250W continuous but 2kW peak... hmmm F1 Mobility Scooter Racing...

But back on-topic, you as a designer can define your own 'continuous'. You might find that your 350W peak motor may not even have the guts to run 100W continuous let alone 250W (which is probably why the manufacturer is embarassingly silent). Typical comparison is the 1kW PEP PC Speaker Systems you see for sale (which run from a dinky 10W wall-wart)!!!!

So if it's a 12V Motor, feed it 12V and then load it untill you get 250W power consumption... come back a week later and see if it's still running or if it's burnt the house down... I'd call that a reasonable 'continuous' test. But really, you have insufficient Data from the manufacturer to even begin any kind of mathematical calculation.