What am I supposed to be doing on my gap year to build my CV?Watch
There should however be some volunteering roles available. Community libraries, charity shops etc. Check your local council for guidance. Things will look better next year and I imagine you will find something before you start your degree.
Also, it's still worth considering Workaway, WWOOF, HelpX etc - you could stay somewhere nice and rural for several weeks, work on a farm or doing DIY, and that way you wouldn't need to worry about the person who's shielding (as you wouldn't live with them any more). The hosts cover your living costs and food, so it's less expensive than traditional travelling, and you could do it within the UK to avoid quarantining.
Other than that, as others here have said things will probably be a bit more normal in a few months, so hang on!