Having a look at the price list again...
XGPON has a £500 fixed connection fee, versus GPON at £92 (£408 difference). Regrade fee is £11.25. I would assume that this difference is the cost of optics.
Even so, £408 + £11.25 = £419.25, which is still cheaper than the difference between a 330 annual fee and the 1000 annual fee (£504) but more than the 160 -> 500mbps annual fee (£406.32).
/methinks this is another small point for your notepad when whispering to OR and getting under the skin of the new FTTPoD regime