No, Cornwall is no better or worse than many places in the UK. My PIL live in Shropshire - beautiful, safe, historic sites, low crime in their town. I live in the Surrey and London - both places are wonderful. National Trust properties, castles, protected forestry sites to walk the dogs, many of the top 20 junior and secondary schools per the times lists (state and private), multiple universities within reach, London under and hour. And London? Vibrant, clean air, culture, marathons, theatre land, art galleries and as safe as any city in the world. I have friends and family in Norwich/Norfolk, Hants etc - they are ALL amazing places. And that’s before you consider parts of Scotland and Wales (Snowdonia national park, Conwy castle OMG stunning), or the northern parts of the uk. Or the Lake District, The Peak District, the Yorkshire dales, York and Harrogate as cultural centres for the arts. And then there is the fact that London/Heathrow is a gateway into the cultural riches of Europe - so easy to access Madrid, Prague, Venice, etc for city breaks. ETA and did I mention the Cathedrals?!!
I’d actually never even consider living in Cornwall, despite how pretty it is - the traffic in tourist season is atrocious, it’s full of drunk teenagers all summer for the festivals and surfing, you are miles from hospitals and amenities as you age, restaurant/Hospitality is stretched due to lack of staff (due to lack of affordable housing for locals) and many businesses pretty much shut down in the autumn/winter as a result. It’s 4hours and £200 by train to London (and therefore the main airports for summer holidays), you can never find a quiet beach in the summer, and every other house is a rental/second home so the local community feel is dying.
TLDR: You have a very warped idea of what the UK is. I’d take the UK over pretty much any country in the world with all its imperfections.