Is it age discrimination to ask the over 70s to continue to self isolate ?

I would say it's that we care about those over 70 since the death rate is much higher in that age group.  

Someone over 70 can choose to stop isolating, but most of us have loved ones over 70 and we want you to remain healthy.  We love you.

But it's your life and your choice.

yes. But not all discrimination is illegal.

People 70 and over in the UK can do what they want.

No, because asking is not law.  The Coronavirus Regulations don't mention it.  And nothing is discrimination if there is a good reason for it, as there is in this case.

If they want a higher chance of dying, then fine, then don't isolate.

We have been asked not told., so no discrimination. It's common sense.

If only asking......then it is not discrimination. 

Not to ask. If they start locking us up, then it's discrimination.