"There is no rule on this. "
That's not true. The emergency regulations remain in force and they continue to restrict the freedom to travel. There isn't a single rule. Each of the four constituent parts of the UK have their own rules.
I believe that what you want to do is allowed between NI and England but would not be allowed (for example) between NI and Wales.
How will you travel from the airport to where you'll see family? Use of public transport in England is restricted to "essential journeys only" and (within next few days) you'll need to use a face covering.