We don't really do levels in the UK. I've heard that some providers will do them, but it isn't the standard in the UK, because support needs change.
To answer your question though, yes I believe it is entirely helpful to get a diagnosis even if it is perceived by others as mild.
I am a late diagnosed adult, you could say that I presented mildly with autism, but mild is how you perceive, not how I live it. However to reach adulthood before receiving a diagnosis, everyone must have perceived my traits as being mild.
I'm an advocate for early diagnosis where ever possible though because along with my adult autism diagnosis, I also received a cPTSD diagnosis.
The way my assessor explained this to me was that you develop cPTSD from a substantial amount of time where your needs are not met.
I had a substantial amount of time in my life where my needs were not met. I didn't even know what my needs were, because nobody bothered to look into an autism diagnosis for me.
Instead I was labelled alsorts of different things: crazy ex-girlfriend material for having meltdowns when plans changed or I couldn't stick to my routines, difficult, demanding, cagey, bitchy for not having typical facial expressions and speaking quite bluntly, smart arse for having a lot of knowledge about certain topics of high interest, know-it-all for again having more knowledge about particular subjects, weird, freak, disgusting etc.
I have been called alsorts of names all of my life, professionally and by being bullied.
I didn't know why when I was at work, and a meeting in my schedule would disappear, it would send me into a spiral, or random teams calls without forewarning would give me panic attacks and I'd not be able to get my words out coherently.
When you think of autism as a spectrum, it isn't linear. It isn't mild to severe. There are obviously some autistic folk that will have persistent lifelong struggles with development and communication. They 100% deserve advocacy, but so do people who's struggles aren't persistent or consistent.
Before I knew I was autistic, there were thinks that I just kept trying to do that other people found easy, and I assumed that everyone found it as hard as I do to do those things but were better at hiding the fact that they found it really hard, which is just not true. Many people do find the things I find hard to do easy and natural to them. This landed me in several bouts of burnout, long term sick, disciplinary action etc. when suddenly receiving a diagnosis opened doors to support through reasonable adjustments at work.
I'd have found it incredibly helpful for school for someone to have realised back then, because the way I learn is different to how other people learn. This meant that years of depression as a teenager might have been alleviated.
Mental health statistics for autistic people are incredibly poor compared to allistic people. The rate of attempted suicides are higher, there becomes an increase in comorbid mental health conditions. The outlooks overall are very grim.
If you're female, the statistics are that about 30% of women in the general population will be sexually assaulted at some point in their life, but when you're autistic and female that figure almost triples. There are a lot of reasons why this can be but ultimately it's because of a social deficit. Whether that's an inability to communicate at all, or whether you understand consent, or whether you have a lack of awareness, or naivety about a situation, and not being able to read others intentions, but not realising you can't read others intentions.
As an autistic person, the chances are that you're more likely to be arrested, because autistic meltdowns can spiral without the right support, but also again, not being able to read others intentions, getting in with the wrong crowd, trying to fit in by doing daft things etc.
If you don't know you're autistic, and you are autistic, you don't really know yourself well enough to safeguard yourself from the above. You don't know that you need to support your mental health in ways that others don't have to, or that you need to be vigilant around the company you keep, and you don't learn techniques or skills to manage intense neurological distress in unavoidable situations, and you don't learn which situations to avoid.
Ultimately, to receive an autism diagnosis, part of the diagnostic criteria is that it has to have a substantial impact on day to day functioning. So again, the only "mild" is how others perceive it, and some autistic folk do perceive their struggles as mild and don't fully realise the substantial disadvantage they are at each day because the model for autism has been heavily based on young white boys with a diagnosis that has only over the past few generations evolved from infantile schizophrenia to where we are now with a better understanding of what it means to be autistic.