The 6-month claim window
When the clock starts
For statutory redundancy, the clock starts on the "relevant date", which is normally your last day of employment (or the day notice expires). The 6-month rule is in ERA 1996 s.164. The tribunal has a discretion to extend by a further 6 months if it would be just and equitable; this is not generous in practice.
The 3-month unfair dismissal clock
Unfair dismissal under ERA 1996 s.98 has a much tighter 3-month-less-one-day window. If you want to argue the redundancy was a sham, or that the selection was unfair, you must claim unfair dismissal in time. Missing the 3-month window kills the unfair dismissal claim regardless of how strong the merits are.
ACAS Early Conciliation
You must notify ACAS of your intent to claim before issuing tribunal proceedings (ACAS EC). The clock is paused for the EC period, then restarts with a minimum of one month to issue. Filing the ET1 is done online at gov.uk employment tribunals.