Pre- departure travel?

Question for UK JETs already in Japan.

For context, I live in the UK but my mum and sibling live in another country. I have dual nationality.

I usually visit my mum, who's quite elderly and unable to travel, in the summer.

However due to preparing for JET departure, I'm wondering how feasible this would be?

If UK departure is late July / early August, would going yo visit my mum for 10 days at the start of July be doable? Do we need to be in the UK at that time? Will we have sent in our passports for visa processing already?

Any advice welcome. Mum is already a bit miffed at me for not doing my usual 3 weeks! 😅