You do realise there are around 1 - 2 million people in UK who are still living in isolation?
This wasn't necessary, pre-covid (for there is still so much we don't know about covid, treatments still not that good, and it's circulating all year round so it's not even the case of a few weeks at height of flu season).
And yes it's shit.
You realise very quickly how disablist society is, when the riskiest part of life for a cancer patient is when they have to go to hospital for treatment because not even there do people practice physical measures like masking and investing in ventilation/filters. And of course now some trusts are even making some of their staff attend workplace when they know they have covid.
It's a really shit life - it's people all ages, and it's everyone on active cancer treatment.
Back to the point of the thread.
OP - unless you are very unlucky, I think it will be behind your household in plenty of time. Plenty of reports that it doesn't last more than a fortnight.
Presumably you've already asked guests with symptoms in their household to keep away, explaining that there will be a highly vulnerable person present (guidelines for confirmed cases say adults 5 days only leaving home if essential to go out, and to keep entirely away from more vulnerable people) and have you asked venue about ventilation?
I'm assuming you're Dad will be masked, and that you've considered how he will eat (does he have a portable hepa filter unit, so indoors near open window, or do you have tables outside under a gazebo sort of thing?)