Do I plead guilty to speeding on the online form, and not guilty to the failure to respond charge? Is that where you’re saying to ‘see if they’ll take that plea’?
Not if you want to avoid the risk of getting nine points.
The police have no evidence that you were driving so the speeding charge cannot succeed (unless you plead guilty to it). The risk (albeit a small one as it doesn’t often happen) is that you will be convicted of speeding (courtesy of your guilty plea) and then be convicted of the “Fail to Provide” (FtP) charge as you have no realistic defence to it.
Assuming your SJPN mentions both the “Fail to Provide” charge and speeding, respond to the Single Justice Procedure Notice (SJPN) by pleading not guilty to both offences. In the "reasons for pleading not guilty" section say you are willing to plead guilty to the speeding offence on the condition that the "failure to provide driver's details" charge is dropped.
Before the pandemic it was necessary to attend court to negotiate this "deal". When the pandemic hit, courts were obviously keen to avoid as many personal attendances as possible and arrangements were made for this deal to be accepted without a court attendance. Many courts have continued with this process but some have reverted to requiring a personal appearance. If you make your intentions clear in response to the SJPN you may be saved a trip.