I no longer have my Z4 but still pop in here from time to time as I still have a great appreciation for these cars (and insurance is insurance). This may help others.
My M2 Competition renewal was due 11 November 2021. Mid October thought I'd take a look at what's coming via a web comparison site.
£370 - pretty good I thought, lower than last year by £50. I'll leave it and see if it comes down any more
(big mistake).
Postal renewal came through end of October- £475: bug*er, but I've still got my £370 deal to go back to.... Wrong.
Checked the website again and the £370 had become £410. Not as good as £370, but certainly better than £475. So I renewed at £410. All conditions on last year's policy carried forward, nothing has changed. Already on max no claims.
Interestingly this was all with my existing insurer - LVE. Renewing early via the comparison site saved me £65,
and LVE recognised me and gave me an existing customer discount

.
Lesson 1, Renew as early as possible via a comparison website.
Lesson 2, Don't take your request to renew at face value.
Lesson 3, Never, Ever, tick the "automatically renew" box on your policy. They'll have your pants down.
Don't know your personal circs Busterboo but from what I used to pay for my 35i your £625 seems a bit on the high side. And I've always found putting your wife on the policy brings the premium down!