My understanding is that EL sold 'Guaranteed Annuities' when interest
rates were high, so annuities were cheap.

When interest rates became low, and Annuities were expensive, EL was
willing and able to fund the 'losses' but refused to pay the same
bonuses to Guaranteed Annuity policy holders as they would to normal
policy holders.

The House of Lords (in its wisdom) decreed that they must pay the same bonuses.
This shafted their current and historical actuarial position.

Could someone put some flesh on those bones (or refute them)

A VERY interesting question, Jerry.

You have tweaked my curiosity!

Bryan