There will be those who use examples like this to say we need such a defence force when arguably there is little reason to expect one, two or more ships to get to the so called invasion in time.
I can see an argument for ships to be deployed around the world for specific conventional warfare or as mobile airstrips but not otherwise. After all the fastest response to a seaborne terrorist attack is likely to be airborne with perhaps enough firepower to do the job.
The headline, as usual with the Mail, is incorrect. One ship cannot protect UK and that is not the main role. It's role is to be at a high state of readiness to deploy anywhere to help in an emergency, terrorist threat or counter drugs. This means that we no longer have any reserves and have no slack left to respond. It also indicates the the Navy has been cut too far.
When there is a need to save money, hard choices often have to be made.
When were we last invaded? There you are then! Easy peasy.
I understand we are still likely to go ahead with a Polaris replacement at enormous cost. The lack of a credible conventional force as a first line of defence means it's deployment gets ever more likely as the only choice in a hostile situation.
As I pointed out earlier this is not only about defence of UK and in fact the ship was more likely to be used in an emergency elsewhere in the world including providing humanitarian support or rescue of British holidaymakers. So it is not easy peasy.
Polaris was replaced by Trident. It is debatable about whether we should replace Trident and whether it should be part of the MOD budget.
I don't really understand your last sentence. Are you trying to say that conventional forces act as a deterrent?