Difficulty with WiFi access in London

If you get a SIM and can unlock your phone, it will work in all three countries. We got one from EE last year in London and used it in several countries.

Most all public Wifi sites require you to agree to a legal disclaimer before you can start using it.
So for example if you were inside a McDonalds and connected to their wifi signal, you then need to open a web browser app, try connecting to a web site (anything like google.com/yahoo.com etc), then you will be redirected to a special McDonalds page where you will a see a disclaimer where you agree not to use the WiFi connection for evil purposes (just click the "agree" box). Once you have done that you can then use the Wifi signal for other internet purposes like checking email, or any apps that require the internet.

android, windows phone or iPhone?

without knowing which phone - try this, if you can. Go into airplane mode. If you have wifi calling on, switch it off until you get connected. Switch on wifi. Depending on your phone, look for wifi networks. Don't connect to any joe blow. Go to a reputable one. When you tap on it if it is secure you will be taken to a login page which you must complete. Or you can download an app.

McDonalds, or Caffe Nero is a good place to start. Watch out for copycats with similar but different names. If in doubt ask behind the counter.

Or go into a carphone warehouse and ask the geek. If they have one.

If I were with you I could help. From what you write I don't have enough information.

A SIM bought in the UK should work in France and Germany. But ask first when you buy at a mobile shop! Vodafone will probably work for roaming but may not be your best deal for data. More info here:

http://prepaid-data-sim-card.wikia.com/wiki/United_Kingdom

Here's some info on unlocking a MetroPCS phone:

https://www.metropcs.com/content/metro/en/desktop/metro/terms-conditions/phone-unlock-policy.html

Most pubs have WiFi and they will give you the password. I’ve never had trouble getting on WiFi on my iPhone when I’m in London or anywhere in the UK. I never buy a SIM card there

Robin, SIM cards are cheap in the UK. Limiting yourself to free WiFi means you can't get public transit and walking direction on the fly with your phone - something that has saved me hours of time and frustration over the last few years while navigating European cities. It doesn't seem worth saving the equivalent of 20-30 euros (or even less) just to avoid buying a SIM.

Thank you everyone for your help. The links were very useful. Carphone was the place someone told me about here so I feel safer using them as you also reccommended them. I have an I phone that I bought with sprint several years ago and it was unlocked when I changed to MetroPCS last year. I wonder if that means it remains unlocked. My husband bought his android from MetroPCS almost a year ago so the link instructions will be very helpful. Thank you. Will go get a SIM card today. As for the agreement page sometimes it automatically showed up and other times not so I will open a web browser to see if that’s my mistake. Thank you all so much.

I have real problem with my phone (Samsung) when a hotspot requires going to a web page. Just doesn't seem to like that even when I try opening a page manually. My wife's phone (LG) doesn't have that issue and can connect many times when I can't

With London, I find free wifi at bookstores, coffee places, restaurants, most hotel lobbies, museums, Barclays, tourist attractions like Kensington Palace, department stores, and pubs among other places. Wifi is definitely unsecured so NEVER go to your bank app or website for example. London for me offers lots of free wifi availability. I travel to London 1-2X per year and have no need at all for a SIM card.

You mentioned roaming. I turn off cell data/roaming as soon as the door closes on my transatlantic flight. I turn it back on when the wheels of my transatlantic flight touch the tarmac at JFK. If you are roaming, you could be in for a horrible surprise on your next cell phone bill. So if you haven't done so, turn off your roaming/cell data ASAP.

I too will be going to Paris next month and then to Germany (near Dortmund). I will not be getting a SIM because among the hotels/coffee places/museums/restaurants/bigger stores, etc I will have access to wifi. I don't want to spend my trip staring at my phone and not at the world at large I've traveled far to see. That's just my POV.

I do know that in Germany, there is a chain of electronic stores called Saturn that sell all types of SIM of cards for whatever data needs and budget you have. The variety available is very good.