The Lifetime Allowance (LTA) is the overall limit a pension plan can reach before its owner is being penalised by a 55% tax upon withdrawal. The Lifetime Allowance after having reached £1,800,000 in 2011/2012 was reduced all the way down to £1,000,000 in 2016/2017 and stayed there for a couple of years before starting to increase again with the CPI (consumer price index) in 2018/2019. It is now at £1,055,000 (2019-2020).
The government tests your pension against the LTA when you take a benefit (eg you take a drawdown or an annuity) or when you reach the age of 75. It's called a benefit crystallisation event (BCE). Nowadays, the most popular way to extract money from a pension is through flexible drawdowns. When you take your benefit via a drawdown, 25% of the drawdown is tax free cash and the rest goes into drawdown mode where it can continue to be reinvested tax free (but still potentially subject to the second LTA test -- see below). The money can be taken at any point from the drawdown fund and is taxed as income on the taxpayer when it is taken. The tax free element and what is left in drawdown is compared to the LTA at the time of the crystallization and the corresponding percentage is added to the ones from the previous drawdowns. If you end up over 100% then the additional amount above the current LTA is either taxed at 55% if you take if out of the drawdown pot immediately or 25% if you leave it there (and it will be taxed a second time as income when you take the money out of the fund).
