RetireJapan

but I decided to apply using my Mum's address in the UK

That might come back to bite you, as it's technically fraud, no?
Imagine winning a large prize, but not being able to claim it because you can't prove you are a resident at the address you claim to live at!
Better to be honest about living in Japan, and just pay the Japanese tax on any winnings.
I read online that for any prizes over 1500 quid, you get sent a letter, and have to actually prove you live at your registered address.
For prizes under 1500 quid, you can just claim online.
(I might be off with the above amount)

Well, I managed to open the account OK and presumably had to prove my address at the time (can't remember the details). I do have some correspondence going to that address, such as bank statements, so maybe that will be enough? But yeah, it's something I should look into.

Prizes over £5000 require a claim form, less than that are paid automatically. And if you win £1,000,000 you get a visit from Agent Million! That could be awkward, yes. I would catch the first flight back to claim that one... but would they still pay it? Not sure.