"I have spoke to 3 law firms briefly. The 1st one said I should instruct them (pay £1,100) and they would speak to the police to get the SJPN dropped - ie would just get 3 points for the ticket"
You can achieve that yourself, as I have explained - and save yourself £1,100. As well as that, you are being misled. The police will not drop the SJPN. It is the only option they now have of enforcing the matter.
The 2nd and 3rd law firm felt confident that if I plead guilty for the speeding ticket and not guilty for the the ‘fail to ID’ offence with the explanation below...
That might work, it might not. It relies on either the police withdrawing the charge following your guilty plea to speeding or, if they don't, the court finding you not guilty based purely on what you have said. There is a possibility (though a remote one) of you being convicted of both charges and receiving nine points. That could happen if you plead guilty to speeding and the police press on with the other charge, which you unsuccessfully defend.
The strategy I have outlined makes that eventuality impossible and gives you a degree of certainty before you enter your pleas. Remember, the speeding prosecution cannot succeed unless you plead guilty to it. So before you help the police out with that, you want something in return.