@Takumi571 , from a conservative approach, I would assume that this is not a qualified plan and therefore include your total wages as part of foreign earned income ( even though the UK is calling it qualified and thus not taxing your contribution) i.e. you expose "X" as your earnings to US taxes. This assures that your basis in the plan is "Y" per year. Therefore when you get distribution from your pension plan , you would be taxed by the US on the growth of Y + Z & its growth. You take the position that this portion is not constructive receipt till distribution. I think that will be safest way to proceed.
If you are willing to take a more aggressive approach ( I really don't see advantage in this ), you claim your total wages is X+Z or X-Y+Z (??) --- the problem with this would be a leaky defense of the constructive receipt principle and when receiving distribution from the plan your basis may be lower but advantage is you will only pay on the growth ( you have to able to prove that if challenged ). It is more complicated and good records needed may be 30 years from now (?) and who knows what the rules would be.
None of this would change UK taxing
