Have you seen this?
When a POP is congested, you get rerouted to other POPs, and that will depend on how much you pay (Free plans first, then Pro, etc).
You won’t necessarily be rerouted to the second nearest POP. I’m in LIS and when this last happened websites were rerouted to either AMS or LRH depending on the ISP. This was pretty common, then it stopped (always get LIS now), which is consistent with LIS getting a capacity upgrade. LHR and AMS are probably the highest capacity (and cheapest to run) European POPs.
Also, not sure if this is still true, but a while ago some POPs never served Free traffic from certain ISPs, because of deals those ISPs failed to strike with Cloudflare (free peering). This was mostly in Australia (also I guess India) but it’s not something I’ve personally witnessed in large scale in Europe.
You get what you pay for. Is the latency that bad to AMS/LHR? Also ZHR doesn’t strike me as bad for FRA traffic.
Finally, if you want traffic being served from the POP closest to your server (not your users), you should enable Argo, and use Argo tunnel. That’ll also cost you.