Hmm, quite the tricky one. Pensions are certainly worth it I reckon, especially if you plan on living for a long time.
National Pensions though are another matter. Both Japan and the UK have aging populations which are putting a strain on their public welfare systems. Currently the Pound-Yen exchange rate is good for sending money home, who knows where it will be in 30 years. Between fallout from Brexit and (in your case) a potential IndyRef2, I'm not sure how beneficial a UK based pension would be. It could be great, but it might not be worth it compared to other options. Depending on how you view potential British governments' chances of successfully managing the country's finances and/or fulfilling their promises is also something to take into account.
Some other things to consider:
How easy will it be to pay every month? How much will you have to pay?
How easy will it be to transfer the money back to Japan (assuming you plan on staying here forever)?
Will the fund you choose be payable to your wife/kids in the event you snuff it early?
Would an international private pension be better than a national public one? I believe that most of the private schemes have insurance in case the company goes under.
Would you be better off investing the money in stocks and shares? (or limited edition minis?)
How about a high interest account?
Are retirement ages going to change?
I'd suggest popping along to somewhere like Houken-no-mado-guchi and getting some information about other options. I went there a few years back and they have a lot of information.
Personally, I have the following pension/insurance plans:
Japanese National Pension
Japanese National Health Insurance
Private Life Insurance - It's a wierd system with an inverse payment system. The sooner I die, the more my wife gets.

Better watch my back. It also pays a fixed amount for my funeral costs.
Private Health Insurance - Covers my stay in hospital for up to 60 days as well as a few thousand yen per day in extra costs. Covers the 30% of medical costs I'd normally pay on the National Health system, though it's only worth claiming on serious bills.
Private Savings Scheme - I pay in a fixed amount per month until I am 65. At any time I can cash out but I have to pay a small penalty (10%ish). If I cash out when I am 65 I get everything I paid in plus a small amount (Maybe 5%ish). However, every month after 65 I wait, the company increases the pot by the same amount I paid prior to being 65. The longer I leave it, the better it gets. It's payable to me or my wife as well. (This is my life-insurance insurance to make sure I don't have to watch my back too closely).
Anyway, lots to think about. Good luck.
TL,DR - The nasty Toaries will steal all your money anyway, so why bother.

Painted Minis in 2014: 510, in 2015: 300, in 2016 :369, in 2019: 417, in 2020: 450