Cellular data roaming-need to turn on here?

Yes you should have "data roaming access" turned on. That allows you to connect to other networks outside of your service provider's network, especially if in Europe.

Yes, turn on roaming. Just keep it on. Do you use Verizon’s international plan? When you do not want to access cellular data, put your phone in airplane mode.

No idea who your service provider is, but here is some information pulled off Verizon's website --

https://www.verizon.com/articles/verizon-unlimited-plans/how-to-minimize-and-avoid-roaming-charges/

https://www.verizon.com/support/international-travel-faqs/#ready-outside-us

How do I get my device ready to use outside the US?
Before you travel internationally, make sure your device's roaming is turned on so your device can connect to cellular networks in your destination country. To turn on roaming, start on your device's home screen:

for iPhone -- Tap Settings (the gear icon).
Tap Cellular, then tap Cellular Data Options, then Roaming.
Slide both the Voice Roaming and the Data Roaming selectors to Green (on).
Slide the International CDMA selector to off.

for Motorola -- Go to your Apps and tap Settings (the gear icon).
Tap Network & Internet, then Mobile Network, then Data Roaming.
Slide the Data Roaming selector to the right until it turns green.
"Allow data roaming?" appears. Choose OK.
Tap Preferred network type, then tap Global.

for Android -- Go to your Apps, tap Settings, then tap More.
Tap Mobile Networks and then Data Roaming access.
Tap Allow access for all trips and Set Network Mode to Global.

We were here in May and I just forgot we had to turn on cellular roaming. So thanks for the reminder that we do.
Yes, we do have the Verizon International Plan.
In the back of my mind I have some idea that you are supposed to avoid data roaming. But that must be for different circumstances I guess?

Thanks VAP for the Verizon article. It was helpful to read it!

Turning on data roaming will mean you incur charges for the data you use whilst in the UK. No doubt at home you have some sort of plan from your phone provider that means that those costs are covered. It's the same here, except that you will need some sort of International plan or you use a local sim with a local plan. If you have an international data plan you can keep data roaming on and you will pay whatever costs are associated with that plan. Without a plan you will be charged walk-up rates, which, if you start browsing the internet, will very rapidly become very costly. Sounds like you're OK though.

One note. If you turn on data roaming which I do, go into your settings for your various apps, and turn off things like background, refresh, etc. I basically turn off the location settings and things like that and use of cellular data by individual app just to avoid something like Gmail constantly pinging and checking for mail using data. And then I check mail on my Wi-Fi at the hotel.

I added my tickets to Notes and am able to access offline. If you add more than one ticket it may look like just one, but tap on it and multiple tickets will show. Hope this helps!

We have Consumer Cellular, and I had them set a max of $100 for roaming for our current trip. They said customers tend to reach that amount quickly. We have been very careful not to use it unless absolutely necessary.

We can manage by keeping phone in airplane mode but turning on wifi. Doing our business while we have wifi access at hotels. Otherwise cellular roaming would cost us $10/day.

This is very much not my field of expertise but it’s perfectly possible to get yourself a local SIM card to give yourselves cheap internet data access. We have done it very successfully a few times in NZ by taking an old phone with us and using that with an NZ sim.