Proof of ID

I have the migration from ESA to UC to look forward to, as well as applying for PIP to replace my indefinite award of DLA.
Being one of those people who neither holds a valid passport or a driving licence, I find myself faced with the problem of providing “acceptable” proof of ID when it comes to accessing the online system of application.
After much thought I came up with the following.
Instead of trying to access the UC system directly, I tried using the HMRC portal on the pretext of wanting to know how much tax I had paid in the last financial year. Armed with my P60, I was able to register adequately and obtain the all-important Govt Gateway Code, which, if my understanding is correct, will give me access to the UC system without having to prove my identity again.....
Has anybody else tried this method? I’m reasonably sure I’ve got my facts straight but I just wondered if anyone could confirm my theory.