I have some thoughts on this. Whilst yes, there is a lot of inefficiency in how the MOD is run, I've done a lot of thought around what our military should be doing. Firstly, you are wrong - the technology we have is ready for modern warfare. Whilst there are jokes based on challenges, on the whole, we continue to have an advanced military capability.
If you compare our circumstances to the countries you've listed, we are an island nation that sits between Russia, Europe and the US. This makes us vulnerable, but also strategic. Without a reasonable military, we would struggle to be allied meaningfully with a friendly force that together operates to deter global super powers. At the moment we are in NATO, which means we are part of an organisation that can represent the interests and operates a deterrent against less-friendly super powers, like China or Russia. We need to be an appealing ally.
With the Navy primarily operate our capability based on deterrent, policing shipping lanes/protecting our interests at sea (oil, fishing), and as a strategic asset that we can operate significant military capability in places where we have no physical land bases. As an island nation reliant on imports, closing of shipping lanes (as we've seen just with oil and the Strait or Hormuz) would be catastrophic to our economy and survival. The Navy support the interests of the UK in global shipping, they also support less military programmes, lots of humanitarian aid, anti-narcotics work and our fisheries protection (and that's protecting us from our allies!).
The nuclear deterrent is something that, if we give up, we can never get back - as that would be a huge escalation on the international stage. At the moment it is part of the baseline threat status as it is out there 24x7 365. Do we need a nuclear deterrent? Arguable, but we have just seen our main ally show a completely different set of colours that may mean at some point Europe needs to rally together to protect itself.
My knowledge is mostly naval, so I'll avoid commenting on our Army and RAF at this point.
Now is not the time to shrink our military further, with unreliable allies and active conflicts between Russia and Ukraine, US and Iran, instability across the middle east. No, we are not a self sufficient nation.
Do we need to review all our government spending? Absolutely, we are inefficient to the nines, we spend far too much on external consultants, contractors etc. We need to do more with our money and get more value out.