If you’re offered ‘IHR with lump sum’ you’d currently get 34 weeks’ pay, plus with 30 years’ service I think 12 weeks PILON.
If you’re offered ‘IHR with immediate payment of pension’ you’d get the pension you’ve accrued up to leaving unreduced. Your annual statement will give you more info.
You would normally only get the pension option if you’re deemed to never be able to work again in any capacity.
*I think IHR payments are on the radar as far as more cost cutting is concerned, so the above might not be the case forever.