Worried about my mums mental health

[Ilovetea09]

My parents are both early 70s, good health. They are usually very active and are never at home. They spend Mon to Fri each week at their caravan in North Wales but live in the Midlands
Due to government advice they are staying in as over 70. Every time I speak to my mum she is sounding more and more down and depressed.
They are hating being stuck in and having nothing to to do, but my mum is also very scared and anxious about actually catching the virus. She's always been very strong mentally and has never worried about illness before so I don't know where this huge fear has come from.
Any advice how I can help her? I know many are in the same position.
She says it's never going to end because there's no vaccine, she's terrified of catching it and not being able to breath, she's missing my 2 children desperately. We speak to her every day. We've also sent her cards and gifts.

In times like these people have plenty for room for reflection. For many this can lead to catastrophizing, following the train of thought until it reaches a "logical" conclusion. Unfortunately in my case this "logical" conclusion always seems to result in my death!

I find the way to combat it is to look at the facts. Not opinions from your neighbour, not even opinions from respectable news outlets.

She says it's never going to end because there's no vaccine
There isn't a vaccine but there probably will be. It takes time but rest assured it is being worked on - even a cynic would accept this, because whoever comes up with the cure will make a fortune.

She's terrified of catching it and not being able to breath
Understandable, but the likelihood of severe problems is relatively small even in at-risk groups. You say she is in "good health" so that would help greatly.

She's missing my 2 children desperately
The only way to avoid catching it is to isolate for the time being. She will see the children again, this situation won't be around forever. In the meantime are they able to speak via Skype or similar? It would probably make her happier seeing them in this way more than just voices on a phonecall.

Her complaint this morning is that they are staying in to protect the NHS and delay infections but that just means when she eventually is allowed to go out she will catch it then as is scared

From what you say, your mum has led a relatively charmed life and enjoyed retirement with the caravan, seeing family, having yourself to turn to when anxious or worried.
Now, that charmed existence is under threat and she can't cope.
Some people are resilient.
Some people understand that life isn't a bowl of cherries all of the time and adapt accordingly.
Some, like our brilliant NHS people put others first.
Your mum needs to get things in perspective OP. Maybe if she focused on something other than herself, her needs, her fears, her anxieties; she'd find a better balance going forward.