Annuity purchase using a broker – timing issues with getting the quoted rate

I’m looking to purchase an annuity and have used an annuity broker to get quotes for an enhanced annuity.

They provided a quoted rate valid for six weeks.  However, the broker says annuities commonly take 8 to 10 weeks to arrange and if my cash doesn’t reach the annuity provider within six weeks, I’ll get whatever the current rate is at the time they get the cash and there is no backing out if I’m not happy with that rate.

Is this something annuity purchasers commonly face?  Is the quoted 8 to 10 weeks partly because of the broker (i.e. I would get it done quicker going direct?).  It doesn’t seem right that I’m being asked to commit to an uncertain annuity.

Do annuity rates potentially change enough for this to be worth worrying about?

Provider will be Aviva.

Thanks for any insights.