We always enjoy the finality of legal process until we are on the losing side. I don't know why UK law is made that way, but nothing should be final, for precisely this reason--clerical errors can happen.
if you never see finality you can worry that some entity will come and change the assumptions at any time.
say that you made a transaction based ona valuation. 15 years later the IRS comes and rewrites that valuation and because this is so late you have absolutely no recourse.