You can use anyone with a professional qualification to sign the form. An Independent Mental Capacity Advisor (IMCA) is the obvious one or an independent social worker and there are others.
Both will do an interview of you both together and alone and if all's well sign the forms.
You can google for an IMCA in your area or check out that, if like the area I live in there is a free service. I have a local drop in centre, LA funded but not part of the NHS or social services that supplies services like this as well as an advocacy service and benefits advise all for free.
Back in the early days the consultant advised me to get an LPA for my wife, I'd no idea what it meant, she explained it all told me where to download the forms and said if I sent them to her she'd sign them. She said this after she'd asked my wife if she was happy for me to look after the money side of things and attend doctors/hospital appointment and the like, my wife agreed, job done. No mention of any charge was ever made so at the time I just assumed that was how it happened, now I know a GP/medical professionals can charge if they want but that doesn't mean they have to.
K