The current UK Corporation Tax structure has a 19% small-profits rate for profits up to £50,000, a 25% main rate above £250,000 and marginal relief between the standard limits. Those thresholds can be reduced for short accounting periods and associated companies.
Why reserve it before the bill arrives?
Because a healthy bank balance can feel spendable months before Corporation Tax is due. A tax reserve gives that future bill a visible job now.