New mattress or new bed?

If you have the funds replace the bed I have a good double now I'm on my own. Half measures wouldn't be my option go for a new bed and have a good bed that will be good for years. A kingsize quilt fits fine on a double too.

Yes, I would go for a new bed. You could still use the same bedding and may even be able to have the headboard adjusted to fit a double. These decisions aren’t easy to make, are they?

If your base is the firm type (ie not sprung) you should be able to put a new mattress on top of it. I'm guessing that if it has drawers this will be the case.
If you decide to buy a new base and the headboard is fixed to the wall it really won't matter how wide the bed is.

I would just replace the mattress if the base itself is still in good condition.

I would just replace the mattress, especially if you really like your existing headboard.
Guidelines will encourage shoppers to spend more money -unless you are 100kg or more, I doubt your bedbase has deteriorated enough to compromise a decent quality new mattress.
Careful with the sizes though, not all "Kingsize" measure the same, get the tape measure out.

We’ve replaced our mattress on both my late mother’s bed (when she was alone and using it!) and our own.

Both were unsprung divans with drawers.

It worked well.