You are permitted 17 mph assisted legally. The DfT have confirmed that the usual 10% tolerance applies to the pedelec assist limit as it does with all other speed limits.
So 15.5 mph + 10% = 17.05 mph
If your system can be set up for that there's no problem.
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You are mixing up guidelines with the law. You are right in saying guidelines have been issued (for motor vehicles) that allow a 10% leeway, plus 3mph, but they are just guidelines and in the news recently was a report that under plans favoured by West Mercia's Chief Constable, drivers could be fined for going just 1mph over the speed limit.
Now we all know the odds of being pulled for doing a couple of MPH over the top are remote, (car, bike or totherwise) but a bored copper on a slow day in an area where there is a crackdown on speeding taking place and said bored copper is wondering why you're going so fast on a push bike and it could happen; particularly if an incident has taken place and they suspect you're e-bike has been modified.
This leads onto a second thought. Exactly which law would the authorities use to charge a cyclist with speeding? As we all know the current speeding laws do not apply to traditional cycles or legal e-bike, but if you've modified your e-bike to have a higher top speed then it's no longer an e-bike, but a moped, and all of a sudden the the law does apply and you're not speeding. See what I'm getting at. Okay you might be up to your neck in it for using an unregistered, untaxed etc bike, but (unless you actually are going above the signed speed limit), the rules have changed.