So long as your helper has their own fully comprehensive car insurance which includes a ‘driving other cars’ (DOC) clause – they’ll be covered to drive yours. However, this will only be on a third party basis which means any damage to your own vehicle will not be covered.
If your helper’s insurance doesn’t carry the DOC clause (check the small print) you’ll need to contact your insurer and temporarily add them to your policy. This can incur an administration fee (typically between £15 and 30) and, again, will only provide third party cover.
If you want your own car to be insured too, you’ll need to add your helper as named driver to your policy (so long as it’s fully comprehensive). This is likely to affect your premium but it in some cases, it could even go down.