Power of attorney

Can I use the Power of Attorney I have for my Father to register his property for this first time? 

Mary - you don't usually need power of attorney to apply to register for the first time if it's a voluntary first registration 
You are I assume compleitng the forms and submitting the application so no 'legal deed' in playt where you are acting as his attorney
First registrations (PG1) - GOV.UK

I am acting as his attorney as he is in a care home and we have discovered his property is not registered  on the land registry. The solicitor is completing the paperwork on my behalf
 

Mary - noted and understood. So it is a voluntary first registration in his own name and based on the deeds/documents that confirm his ownership. There's nothing in such an application that would require completion by his attorney but I'm sure the solicitor will confirm. 

We have been told that The Land Registry require either a letter from Drs stating he hasn't capacity or a letter signed by him giving me authority to act on his behalf. I thought this was what the LPA was for!
 

Mary - an LPA provides a lasting power of attorney but the process for it coming into play will invariably rely on a Doctor confirming the donor has indeed lost mental capacity. 

No. An LPA can be used either when capacity has been lost or immediately, as in our case, depending on what was chosen on the LPA by the donor. This allows it to be used to help the donor even when they still have capacity but are frail and just need help due to other issues such as deafness, speech impairments etc

Mary - true but your Q is really aimed at whoever has told you what you need, not HMLR. If it's a first registration in the name of the donor then the attorney has no direct role unless of course the legal rep is advising you otherwise. 
That role might for example come into play of dealing with the actual property such as selling but that's not what you appear to be suggesting 
If you are concerned re the legal advice given go back and ask them to refer you to the guidance on why such an assessment is required.