Or is it quite difficult to do this as an amateur in the UK?

No input?

Why would it be difficult? There are /loads/ of charities, care homes, social groups around that I'm sure would love some entertainment, especially f you are volunteering for free. Either pick a cause that's close to your heart and seek out local groups for that, or contact your local council who will have a list of various socail groups in your areas. It might even be on their website.

What do people find works in different venues? Eg an old folks home -Walkaround closeup or 'putting on a show'?

I guess the biggest obstacle would be safeguarding policies however if this can be overcome easy enough with an up to date DBS check.

Many places would be welcome of you volunteering your time in this way. It's easily done.

First you would have to get a DBS clearance police check.
This would enable you to work in sensitive areas like hospitals, schools etc.

Standard procedure.