The regs may not reflect it right now but the govt guidance now does say clearly that a single person "bubbling"
can stay overnight:
https://www.gov.uk/guidance/meeting-people-from-outside-your-household. It says:
"...from 13 June, single adult households – in other words adults who live alone or with dependent children only – can form a ‘support bubble’ with one other household. All those in a support bubble will be able to spend time together inside each others’ homes,
including overnight, without needing to stay 2 metres apart."
Under the regs that would as far as I can see be interpreted as "reasonable excuse" as it's hard to see how following the explicit govt guidance could not be judged as reasonable.