Internet data transfer in populated areas/big city

[Ionutbuse]

The O2 telecommunication network doesn't respect the contract terms, and also me as a customer.
I am very unhappy with this network. Yesterday I was unable to enjoy the Internet in the most important and tourist areas of London. And not only in London, is happen in Manchester or Liverpool, is the same. I went directly to the O2 shop, I explained, that there it is and it show on my phone Huawei Honor 9, maximum signal 4G, but extremely low speed Internet, I needed 5 minutes to upload 2 photos on Facebook, near London Eye, I've try to call my parents from my country with video call on WhatsApp or Facebook Messenger, I couldn't do that. Including they FROM THE O2 SHOP said to me it is frustrating. I told them I want to end the contract right now with them ....
Their answer: "you will be charge 20£ if you want to end the contract"
My question: "If I can't use your service... How can I charge you than?"

Went many times to O2 shops and discuss this problem with them.
Why there is just excuses but nothing to do. I'm basically trapped to O2, can't enjoy Internet in my free time when I visiting tourist places.
Here is my questions:

Is it there a way to end contract without being charged?
If O2 doesn't respect the contract, why I can't end the contract?
I'm feel like in "prison" with O2, being innocent customer who can't use their service. How can I get out from here?

[gmarkj]

Completely down to the amount of traffic the o2 network can cope with.
It has been commented on before that major cities suffer from a noticably slower speed than other networks.
What might help (if you are still there and you can) is to switch back to 3G as that may be faster.

[Ionutbuse]

Bambino
I did mention in my first post, there is maximum signal on my phone, and shows me 4G. But no data transfer, if you can understand what I mean. I've change my phone from Samsung S6 to Huawei honor 9, new.
Yes I've change sim twice.
Far to believe O2 are best.
And again, as a O2 customer is unacceptable to don't enjoy data in big cities. I have no problems anywhere else, only in big cities, when I should enjoy my days off, free time.
Congestion? Is not my problem. Why should we suffer because O2 networks crushes when many people use 4G? In the contract is not specify that.
Not the same for Virgin, when I saw with my eye on my friend phone in same places.
Thanks all for reply and advices.

[madasaf1sh]

Mobile coverage on all Networks is patchy at best in Central, one network can have awesome coverage in one area and then none in another,

There is another issue at the moment in Central London, that there is a lot of infrastructure work going on for crossrail and others, which has taken out some of the backbones and cell sites, indeed on Oxford Street and Tottenham Court Road, there no works allowed until the CrossRail infrastructure is finished, so none of the networks can put up any masts or run any cables.. so the coverage...
EE/BT who Virgin use, probably have mast close by...

THe contract states that Network access is not guaranteed.